ADS7924: Not getting the proper converted data Via I2C

Balaji TS2

Part Number: ADS7924
Other Parts Discussed in Thread: TM4C129DNCPDT, , AMC1301, INA333

Tool/software:

Grretings for the day,

We are using TM4C129DNCPDT microcontroller to read ADS7924 converted data using I2C protocol. But we are not getting proper data.

We have verified I2C communication by reading Device Id and mode settings. They are reading properly.

When have connected Constant 3Vpp Input voltage using Function generator to ADC channel CH0, we are getting different values everytime we read 0x02 and 0x03 registers. We have combined both register readings to get 12 bit digital data and converted to voltage format using below formula. Voltage output is varieng between 0.5V to 3V continously.

Volt = (12_bit_data/ 4095) * 3.3; //AVDD connected to 3.3V

But when we remove input at CH0, we are getting constant 0x61 from 0x02 register and variable value from 0x03 register. After implementing the formula the voltage is coming between (1.269v to 1.272v).

We are following the below steps to configure ADC.

1. Write 0xAA to 0x16 register to reset ADC.

2. Write 0x80 to 0x00 register to keep ADC in awake mode.

3. Write 0xC0 to 0x00 register to Select ADC CH0 Manual Single mode.

4. Now to read CH0 converted data, In I2C write mode we are sending 0x82 to ADC and changing the mode to burst read mode and reading twice so that we will get 0x02 and 0x03 register data respectively.

5. Now we are continously repeating step 3 and 4 to read ADC converted data continously but we are getting different data everytime(Input Voltage is constatnt 3Vpp).

Are we following the steps properly? or Is there anything we are missing? If so please correct us.

Our Schematics screenshot:

Thank You.

Regards,

Balaji TS

2 months ago

0 Tom Hendrick 2 months ago

TI__Guru**** 180831 points

Hi Balaji,

Please describe what is to the right of the analog inputs. Do you have a buffer between the ADC input and function generator?

0 Balaji TS2 2 months ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

Thank You for the quick response.

We have a Gain Isolation Amplifier Circuit(AMC1301) and a Precision Amplifier (INA333AIDGKR) between ADC input and Function Generator. Currently I don't have full access to schematics. I got above information from our hardware team.

Is buffer compulsory between ADC input and Function Generator?

Regards,

Balaji TS

0 Tom Hendrick 2 months ago in reply to Balaji TS2

TI__Guru**** 180831 points

Hi Balaji,

It depends - there are cases where you can get away without a buffer in front of a SAR converter like the ADS7924. Can you use an o'scope and verify that the amplifier is not oscillating? Take a look at this video and see if it helps you resolve the issue: https://www.youtube.com/watch?v=JWmSgP0KQWQ

0 Balaji TS2 2 months ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

Sorry for the late response.

The issue we were facing was due to Analog input which we were giving. By mistakenly we gave AC Voltage to ADS7924 input channels. Which caused that issue.

Now we are giving input using DC Voltage source, now we are able to read proper ADC output but there is around 100mV continous fluctuaions in the readings. I have attached ADC readings which conatins register readings and converted Voltage values for 1V, 2V and 3V input below. Can you please tell us what might be causing the issue?

Fullscreen 1V Readings.txt Download

MSB data[0]=0x71, LSB data[1]=0xc0
Analog input voltage = 1.000V ADC output Voltage = 1.0171V

MSB data[0]=0x72, LSB data[1]=0xf0
Analog input voltage = 1.000V ADC output Voltage = 1.0887V

MSB data[0]=0x72, LSB data[1]=0xe0
Analog input voltage = 1.000V ADC output Voltage = 1.0849V

MSB data[0]=0x72, LSB data[1]=0x20
Analog input voltage = 1.000V ADC output Voltage = 1.0397V

MSB data[0]=0x71, LSB data[1]=0x20
Analog input voltage = 1.000V ADC output Voltage = 0.9795V

MSB data[0]=0x72, LSB data[1]=0x70
Analog input voltage = 1.000V ADC output Voltage = 1.0586V

MSB data[0]=0x71, LSB data[1]=0x40
Analog input voltage = 1.000V ADC output Voltage = 0.9870V

MSB data[0]=0x71, LSB data[1]=0xf0
Analog input voltage = 1.000V ADC output Voltage = 1.0284V

MSB data[0]=0x72, LSB data[1]=0x80
Analog input voltage = 1.000V ADC output Voltage = 1.0623V

MSB data[0]=0x72, LSB data[1]=0x40
Analog input voltage = 1.000V ADC output Voltage = 1.0473V

MSB data[0]=0x72, LSB data[1]=0x20
Analog input voltage = 1.000V ADC output Voltage = 1.0397V

MSB data[0]=0x71, LSB data[1]=0x60
Analog input voltage = 1.000V ADC output Voltage = 0.9945V

MSB data[0]=0x72, LSB data[1]=0x00
Analog input voltage = 1.000V ADC output Voltage = 1.0322V

MSB data[0]=0x71, LSB data[1]=0x80
Analog input voltage = 1.000V ADC output Voltage = 1.0021V

MSB data[0]=0x71, LSB data[1]=0x20
Analog input voltage = 1.000V ADC output Voltage = 0.9795V

MSB data[0]=0x72, LSB data[1]=0xe0
Analog input voltage = 1.000V ADC output Voltage = 1.0849V

MSB data[0]=0x72, LSB data[1]=0x40
Analog input voltage = 1.000V ADC output Voltage = 1.0473V

MSB data[0]=0x72, LSB data[1]=0x90
Analog input voltage = 1.000V ADC output Voltage = 1.0661V

MSB data[0]=0x72, LSB data[1]=0x50
Analog input voltage = 1.000V ADC output Voltage = 1.0510V

MSB data[0]=0x71, LSB data[1]=0x80
Analog input voltage = 1.000V ADC output Voltage = 1.0021V

MSB data[0]=0x71, LSB data[1]=0x50
Analog input voltage = 1.000V ADC output Voltage = 0.9908V

MSB data[0]=0x71, LSB data[1]=0xa0
Analog input voltage = 1.000V ADC output Voltage = 1.0096V

MSB data[0]=0x72, LSB data[1]=0xe0
Analog input voltage = 1.000V ADC output Voltage = 1.0849V

MSB data[0]=0x71, LSB data[1]=0x30
Analog input voltage = 1.000V ADC output Voltage = 0.9832V

MSB data[0]=0x71, LSB data[1]=0x10
Analog input voltage = 1.000V ADC output Voltage = 0.9757V

MSB data[0]=0x72, LSB data[1]=0x20
Analog input voltage = 1.000V ADC output Voltage = 1.0397V

MSB data[0]=0x72, LSB data[1]=0x90
Analog input voltage = 1.000V ADC output Voltage = 1.0661V

MSB data[0]=0x71, LSB data[1]=0x70
Analog input voltage = 1.000V ADC output Voltage = 0.9983V

MSB data[0]=0x71, LSB data[1]=0xd0
Analog input voltage = 1.000V ADC output Voltage = 1.0209V

MSB data[0]=0x72, LSB data[1]=0x00
Analog input voltage = 1.000V ADC output Voltage = 1.0322V

MSB data[0]=0x72, LSB data[1]=0x00
Analog input voltage = 1.000V ADC output Voltage = 1.0322V

MSB data[0]=0x71, LSB data[1]=0x10
Analog input voltage = 1.000V ADC output Voltage = 0.9757V

MSB data[0]=0x71, LSB data[1]=0x90
Analog input voltage = 1.000V ADC output Voltage = 1.0058V

MSB data[0]=0x72, LSB data[1]=0xd0
Analog input voltage = 1.000V ADC output Voltage = 1.0812V

MSB data[0]=0x72, LSB data[1]=0xf0
Analog input voltage = 1.000V ADC output Voltage = 1.0887V

Fullscreen 2V Readings.txt Download

MSB data[0]=0x83, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 2.0565V

MSB data[0]=0x82, LSB data[1]=0x70
Analog input voltage = 2.000V ADC output Voltage = 2.0226V

MSB data[0]=0x82, LSB data[1]=0x10
Analog input voltage = 2.000V ADC output Voltage = 2.0000V

MSB data[0]=0x82, LSB data[1]=0x30
Analog input voltage = 2.000V ADC output Voltage = 2.0075V

MSB data[0]=0x82, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 1.9962V

MSB data[0]=0x82, LSB data[1]=0xe0
Analog input voltage = 2.000V ADC output Voltage = 2.0489V

MSB data[0]=0x82, LSB data[1]=0x60
Analog input voltage = 2.000V ADC output Voltage = 2.0188V

MSB data[0]=0x82, LSB data[1]=0x90
Analog input voltage = 2.000V ADC output Voltage = 2.0301V

MSB data[0]=0x83, LSB data[1]=0xb0
Analog input voltage = 2.000V ADC output Voltage = 2.0979V

MSB data[0]=0x82, LSB data[1]=0xb0
Analog input voltage = 2.000V ADC output Voltage = 2.0376V

MSB data[0]=0x82, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 1.9962V

MSB data[0]=0x82, LSB data[1]=0xc0
Analog input voltage = 2.000V ADC output Voltage = 2.0414V

MSB data[0]=0x83, LSB data[1]=0x90
Analog input voltage = 2.000V ADC output Voltage = 2.0904V

MSB data[0]=0x81, LSB data[1]=0xb0
Analog input voltage = 2.000V ADC output Voltage = 1.9774V

MSB data[0]=0x82, LSB data[1]=0xe0
Analog input voltage = 2.000V ADC output Voltage = 2.0489V

MSB data[0]=0x83, LSB data[1]=0xd0
Analog input voltage = 2.000V ADC output Voltage = 2.1054V

MSB data[0]=0x83, LSB data[1]=0x90
Analog input voltage = 2.000V ADC output Voltage = 2.0904V

MSB data[0]=0x82, LSB data[1]=0xa0
Analog input voltage = 2.000V ADC output Voltage = 2.0339V

MSB data[0]=0x82, LSB data[1]=0xb0
Analog input voltage = 2.000V ADC output Voltage = 2.0376V

MSB data[0]=0x82, LSB data[1]=0x40
Analog input voltage = 2.000V ADC output Voltage = 2.0113V

MSB data[0]=0x82, LSB data[1]=0xc0
Analog input voltage = 2.000V ADC output Voltage = 2.0414V

MSB data[0]=0x82, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 1.9962V

MSB data[0]=0x82, LSB data[1]=0xb0
Analog input voltage = 2.000V ADC output Voltage = 2.0376V

MSB data[0]=0x83, LSB data[1]=0x70
Analog input voltage = 2.000V ADC output Voltage = 2.0828V

MSB data[0]=0x82, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0263V

MSB data[0]=0x82, LSB data[1]=0x10
Analog input voltage = 2.000V ADC output Voltage = 2.0000V

MSB data[0]=0x82, LSB data[1]=0x30
Analog input voltage = 2.000V ADC output Voltage = 2.0075V

MSB data[0]=0x82, LSB data[1]=0xd0
Analog input voltage = 2.000V ADC output Voltage = 2.0452V

MSB data[0]=0x82, LSB data[1]=0x70
Analog input voltage = 2.000V ADC output Voltage = 2.0226V

MSB data[0]=0x82, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 1.9962V

MSB data[0]=0x82, LSB data[1]=0x40
Analog input voltage = 2.000V ADC output Voltage = 2.0113V

MSB data[0]=0x82, LSB data[1]=0x50
Analog input voltage = 2.000V ADC output Voltage = 2.0150V

MSB data[0]=0x82, LSB data[1]=0x90
Analog input voltage = 2.000V ADC output Voltage = 2.0301V

MSB data[0]=0x82, LSB data[1]=0xf0
Analog input voltage = 2.000V ADC output Voltage = 2.0527V

MSB data[0]=0x83, LSB data[1]=0x30
Analog input voltage = 2.000V ADC output Voltage = 2.0678V

MSB data[0]=0x82, LSB data[1]=0xa0
Analog input voltage = 2.000V ADC output Voltage = 2.0339V

MSB data[0]=0x82, LSB data[1]=0x70
Analog input voltage = 2.000V ADC output Voltage = 2.0226V

MSB data[0]=0x82, LSB data[1]=0xd0
Analog input voltage = 2.000V ADC output Voltage = 2.0452V

MSB data[0]=0x83, LSB data[1]=0x90
Analog input voltage = 2.000V ADC output Voltage = 2.0904V

MSB data[0]=0x83, LSB data[1]=0x10
Analog input voltage = 2.000V ADC output Voltage = 2.0602V

MSB data[0]=0x83, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0866V

MSB data[0]=0x82, LSB data[1]=0xd0
Analog input voltage = 2.000V ADC output Voltage = 2.0452V

MSB data[0]=0x82, LSB data[1]=0x60
Analog input voltage = 2.000V ADC output Voltage = 2.0188V

MSB data[0]=0x82, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0263V

MSB data[0]=0x82, LSB data[1]=0x20
Analog input voltage = 2.000V ADC output Voltage = 2.0037V

MSB data[0]=0x83, LSB data[1]=0xe0
Analog input voltage = 2.000V ADC output Voltage = 2.1092V

MSB data[0]=0x82, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0263V

MSB data[0]=0x82, LSB data[1]=0x60
Analog input voltage = 2.000V ADC output Voltage = 2.0188V

MSB data[0]=0x82, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 1.9962V

MSB data[0]=0x82, LSB data[1]=0xc0
Analog input voltage = 2.000V ADC output Voltage = 2.0414V

MSB data[0]=0x82, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0263V

MSB data[0]=0x83, LSB data[1]=0x30
Analog input voltage = 2.000V ADC output Voltage = 2.0678V

MSB data[0]=0x82, LSB data[1]=0x30
Analog input voltage = 2.000V ADC output Voltage = 2.0075V

MSB data[0]=0x82, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0263V

MSB data[0]=0x83, LSB data[1]=0x10
Analog input voltage = 2.000V ADC output Voltage = 2.0602V

MSB data[0]=0x82, LSB data[1]=0x10
Analog input voltage = 2.000V ADC output Voltage = 2.0000V

MSB data[0]=0x82, LSB data[1]=0x20
Analog input voltage = 2.000V ADC output Voltage = 2.0037V

MSB data[0]=0x82, LSB data[1]=0xe0
Analog input voltage = 2.000V ADC output Voltage = 2.0489V

MSB data[0]=0x82, LSB data[1]=0x00
Analog input voltage = 2.000V ADC output Voltage = 1.9962V

MSB data[0]=0x82, LSB data[1]=0xf0
Analog input voltage = 2.000V ADC output Voltage = 2.0527V

MSB data[0]=0x82, LSB data[1]=0x20
Analog input voltage = 2.000V ADC output Voltage = 2.0037V

MSB data[0]=0x82, LSB data[1]=0x70
Analog input voltage = 2.000V ADC output Voltage = 2.0226V

MSB data[0]=0x82, LSB data[1]=0x90
Analog input voltage = 2.000V ADC output Voltage = 2.0301V

MSB data[0]=0x83, LSB data[1]=0x80
Analog input voltage = 2.000V ADC output Voltage = 2.0866V

MSB data[0]=0x82, LSB data[1]=0xf0
Analog input voltage = 2.000V ADC output Voltage = 2.0527V

MSB data[0]=0x83, LSB data[1]=0xa0
Analog input voltage = 2.000V ADC output Voltage = 2.0941V

Fullscreen 3V Readings.txt Download

MSB data[0]=0x93, LSB data[1]=0xc0
Analog input voltage = 3.000V ADC output Voltage = 3.0657V

MSB data[0]=0x93, LSB data[1]=0xc0
Analog input voltage = 3.000V ADC output Voltage = 3.0657V

MSB data[0]=0x93, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0619V

MSB data[0]=0x93, LSB data[1]=0x40
Analog input voltage = 3.000V ADC output Voltage = 3.0356V

MSB data[0]=0x93, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0280V

MSB data[0]=0x93, LSB data[1]=0x50
Analog input voltage = 3.000V ADC output Voltage = 3.0393V

MSB data[0]=0x93, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0280V

MSB data[0]=0x93, LSB data[1]=0x30
Analog input voltage = 3.000V ADC output Voltage = 3.0318V

MSB data[0]=0x92, LSB data[1]=0x70
Analog input voltage = 3.000V ADC output Voltage = 2.9866V

MSB data[0]=0x92, LSB data[1]=0x50
Analog input voltage = 3.000V ADC output Voltage = 2.9791V

MSB data[0]=0x92, LSB data[1]=0xe0
Analog input voltage = 3.000V ADC output Voltage = 3.0130V

MSB data[0]=0x92, LSB data[1]=0x50
Analog input voltage = 3.000V ADC output Voltage = 2.9791V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0xa0
Analog input voltage = 3.000V ADC output Voltage = 3.0581V

MSB data[0]=0x93, LSB data[1]=0xd0
Analog input voltage = 3.000V ADC output Voltage = 3.0694V

MSB data[0]=0x93, LSB data[1]=0x10
Analog input voltage = 3.000V ADC output Voltage = 3.0243V

MSB data[0]=0x93, LSB data[1]=0x60
Analog input voltage = 3.000V ADC output Voltage = 3.0431V

MSB data[0]=0x93, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0280V

MSB data[0]=0x93, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0280V

MSB data[0]=0x93, LSB data[1]=0x90
Analog input voltage = 3.000V ADC output Voltage = 3.0544V

MSB data[0]=0x93, LSB data[1]=0xf0
Analog input voltage = 3.000V ADC output Voltage = 3.0770V

MSB data[0]=0x93, LSB data[1]=0xe0
Analog input voltage = 3.000V ADC output Voltage = 3.0732V

MSB data[0]=0x93, LSB data[1]=0x60
Analog input voltage = 3.000V ADC output Voltage = 3.0431V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0619V

MSB data[0]=0x93, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0619V

MSB data[0]=0x93, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0619V

MSB data[0]=0x93, LSB data[1]=0xc0
Analog input voltage = 3.000V ADC output Voltage = 3.0657V

MSB data[0]=0x93, LSB data[1]=0x50
Analog input voltage = 3.000V ADC output Voltage = 3.0393V

MSB data[0]=0x93, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0619V

MSB data[0]=0x92, LSB data[1]=0x90
Analog input voltage = 3.000V ADC output Voltage = 2.9941V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x92, LSB data[1]=0xc0
Analog input voltage = 3.000V ADC output Voltage = 3.0054V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0x30
Analog input voltage = 3.000V ADC output Voltage = 3.0318V

MSB data[0]=0x93, LSB data[1]=0x70
Analog input voltage = 3.000V ADC output Voltage = 3.0468V

MSB data[0]=0x93, LSB data[1]=0xf0
Analog input voltage = 3.000V ADC output Voltage = 3.0770V

MSB data[0]=0x94, LSB data[1]=0x70
Analog input voltage = 3.000V ADC output Voltage = 3.1071V

MSB data[0]=0x93, LSB data[1]=0xd0
Analog input voltage = 3.000V ADC output Voltage = 3.0694V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0xc0
Analog input voltage = 3.000V ADC output Voltage = 3.0657V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0x40
Analog input voltage = 3.000V ADC output Voltage = 3.0356V

MSB data[0]=0x93, LSB data[1]=0xc0
Analog input voltage = 3.000V ADC output Voltage = 3.0657V

MSB data[0]=0x92, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0017V

MSB data[0]=0x93, LSB data[1]=0x30
Analog input voltage = 3.000V ADC output Voltage = 3.0318V

MSB data[0]=0x93, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0280V

MSB data[0]=0x92, LSB data[1]=0xf0
Analog input voltage = 3.000V ADC output Voltage = 3.0167V

MSB data[0]=0x94, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0883V

MSB data[0]=0x93, LSB data[1]=0x40
Analog input voltage = 3.000V ADC output Voltage = 3.0356V

MSB data[0]=0x93, LSB data[1]=0x00
Analog input voltage = 3.000V ADC output Voltage = 3.0205V

MSB data[0]=0x93, LSB data[1]=0xb0
Analog input voltage = 3.000V ADC output Voltage = 3.0619V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

MSB data[0]=0x93, LSB data[1]=0x60
Analog input voltage = 3.000V ADC output Voltage = 3.0431V

MSB data[0]=0x93, LSB data[1]=0x30
Analog input voltage = 3.000V ADC output Voltage = 3.0318V

MSB data[0]=0x93, LSB data[1]=0x70
Analog input voltage = 3.000V ADC output Voltage = 3.0468V

MSB data[0]=0x93, LSB data[1]=0x30
Analog input voltage = 3.000V ADC output Voltage = 3.0318V

MSB data[0]=0x93, LSB data[1]=0xf0
Analog input voltage = 3.000V ADC output Voltage = 3.0770V

MSB data[0]=0x93, LSB data[1]=0x30
Analog input voltage = 3.000V ADC output Voltage = 3.0318V

MSB data[0]=0x93, LSB data[1]=0x20
Analog input voltage = 3.000V ADC output Voltage = 3.0280V

MSB data[0]=0x93, LSB data[1]=0x80
Analog input voltage = 3.000V ADC output Voltage = 3.0506V

Thank You.

Regard,

Balaji TS

0 Tom Hendrick 2 months ago in reply to Balaji TS2

TI__Guru**** 180831 points

Short your input and see what sort of conversion results you are getting. Can you include the full schematic for the input stage please?

0 Balaji TS2 1 month ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

Sorry for the late response. Today we got access to the schemtics of our board. Below I have attached the schematic for the input stage. Please check it once.

Tom Hendrick said:
Short your input and see what sort of conversion results you are getting

What do you mean short the input? Do we have to short input side positive and negative pins?

Thank You.

Regards,

Balaji TS

0 Tom Hendrick 1 month ago in reply to Balaji TS2

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Short each channel to ground (one at a time, using tweezers for instance) at or as close as you can get to the ADC inputs and check the conversion results. Then go back through the rest of your analog front end and do the same thing. Short the output of the INA, then the input - the output of the ISO AMP, etc. That can help you to locate where the noise is coming from.

0 Balaji TS2 1 month ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

We have probed at few places in board, we have observed some noice at all the points and below attached the DSO waveform of the same.

We have probed at the highlighed points in the schematics.

1. At pads of C941(DNP) - observed 232mV continous dip in the voltage.

So we are planning to do the following reworks:- i) Mounting C941 with 0.1uF cap to eliminate noise. ii) Add Bulk decap of 10uF for U15 (INA333).

2. At R2310 Both pads - observed 86mV noise

3. At D137 Both pads - observed 48mV noise

Any suggestions from your end regarding the scope probing or rework?

Thank You.

Regards,

Balaji TS

0 Tom Hendrick 1 month ago in reply to Balaji TS2

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Hi Balaji,

That blip at C91 is probably caused by the charging of the internal sample/hold capacitor. Can you probe C94 as well?

0 Balaji TS2 1 month ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

We have probed at C94 capacitor. and we applied analog input 1V while probing at C94.

Regards,

Balaji

0 Tom Hendrick 1 month ago in reply to Balaji TS2

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Is that from C941 at the output of the INA or C94 at the ADC MUXOUT pin? Is this just one channel sampling over and over or is it multiple channels?

0 Balaji TS2 1 month ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

This probing is at C94 ADC MUXOUT pin. And we are sampling only one channel continously using Auto Single mode.

Regards,
Balaji TS

0 Tom Hendrick 1 month ago in reply to Balaji TS2

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You can try reducing the value of R1 and C94. Here is another video that talks about driving the input of a SAR converter. C94 would typically be 20 to 60X the value of the internal sample/hold capacitor, which is 15pF. Your 1nF is on the high side. The other thing you can try is slowing down the throughput of the ADC - basically give the input more time to settle before starting the conversion. Here is the link: https://www.ti.com/video/5466215483001 this is another good introduction: https://www.ti.com/video/5476574757001

0 Balaji TS2 1 month ago in reply to Tom Hendrick

Intellectual 260 points

Hello Tom,

Sorry for the late response, We have tested by reducing the R1 and C94 values, but the issue didn't resolve. Still the ADC output was varieing. So we have reversed all the rework and changed the mode from Auto Scan mode to Manual Single mode, now there is bit improvements in the readings. Previously we were observing variatinon upto 150mV but not the variations reduced 30mV if the input applied is 3V.

And also we have current input applied to ADC, where we will be providing 4mA to 20mA. In this convertion the LSB bits are varieing a lot, after calculating the current we are observing upto 0.1mA variations. Below is the schematic for current input stage.

AINC1_P is connected CH_2 of ADC module. Is there any issue in our schematic, why is the current readings are varieing so much?

Thank You.

Regards,

Balaji TS

0 Tom Hendrick 1 month ago in reply to Balaji TS2

TI__Guru**** 180831 points

Hi Balaji,

Are you still seeing those large transients at C94 on the MUXOUT pin? Are you willing.able to try adding a buffer there, similar to what is shown in Figure 62?

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