ADS1278: How to measure 5V Analog IN using ADS1278

Jayanthan V

Prodigy 25 points

Part Number: ADS1278

What is the maximum input analog voltage that can be measured using ADS1278. Please find the attached schematic.

14 days ago

0 Bryan Lizon86 12 days ago

TI__Mastermind 35641 points

Hi Jayanthan V,

Please consult the ADC datasheet for this type of information:

-Bryan

0 Jayanthan V 12 days ago

Prodigy 25 points

But I can read only 1/2 of the vref given. I.e., 1.25V when 2.5V or above analog input is applied.

0 Bryan Lizon86 12 days ago in reply to Jayanthan V

TI__Mastermind 35641 points

Hi Jayanthan V,

If you are using a 2.5V reference, then the maximum full-scale input voltage you can measure is VREF = 2.5V. If you apply a differential signal >2.5V, then the output code will just be the ADC max positive code (7FFFFFh)

Also, the ADS1278 has a binary twos complement coding scheme, so half of the codes are for the positive range (000000h to 7FFFFFh) and half of the codes are for the negative range (800000h to 000000h). If you are getting 7FFFFFh when you apply a signal >2.5V, the ADC is operating as intended

All of this information is in the datasheet. Please review this document in detail so you understand how the ADC actually works

-Bryan

0 Jayanthan V 11 days ago in reply to Bryan Lizon86

Prodigy 25 points

Hello Bryan,

Kindly, help me out in understanding what is the maximum analog input voltage that can be measured with the above attached schematic.

0 Bryan Lizon86 11 days ago in reply to Jayanthan V

TI__Mastermind 35641 points

Hi Jayanthan V,

As mentioned, this information is included in the ADC datasheet. I even provided a picture of the relevant section

Nothing that I can see in your schematic changes these requirements.

-Bryan

0 Jayanthan V 11 days ago in reply to Bryan Lizon86

Prodigy 25 points

Hi Bryan,

But my output saturates to 7FFFF , when input is more than 1.25V

0 Bob Benjamin 11 days ago in reply to Jayanthan V

TI__Guru** 113335 points

Hi Jayanthan V,

Currently our ADS1278 experts are out of the office until next week so be patient for a complete response. The ADS1278 is very low input impedance so you should have a driver amplifier in front of your inputs. It appears that you are directly connecting the input to a source. A more complete schematic would be helpful. Also oscilloscope shots of the input voltage would help us determine why you seeing full-scale. Most likely the input voltage is exceeding the + full-scale range. If your AC input is a sine wave (for example) centered at 1.25V, then exceeding 1.25V would reach full-scale (1.25 + 1.25 = 2.5).

See figure 90 in the datasheet for a basic single-ended interface.

Best regards,

Bob B

0 Jayanthan V 10 days ago in reply to Bob Benjamin

Prodigy 25 points

Hello Bob Benjamin,

I have signal conditioning circuit before connecting to ADC.

For instance, I have provided input from Regulated power supply. Input Voltage is in control, so there is no chance of exceeding the limit.

0 Bob Benjamin 10 days ago in reply to Jayanthan V

TI__Guru** 113335 points

Hi Jayanthan V,

As I said in my previous post, the ones most familiar with the operation of this part are currently out of the office. You will have to wait until next week to get any additional help.

Best regards,

Bob B

0 Jayanthan V 4 days ago in reply to Bob Benjamin

Prodigy 25 points

Hi Bob Benjamin,

Is there any update?

+1 Keith Nicholas 2 days ago in reply to Jayanthan V

TI__Mastermind 36710 points

Hello Jayanthan,

When using a single-ended configuration as you have shown (AINN=GND), and VREF=2.5V, the maximum full scale input voltage on AINP will be 2.5V. With 2.5V, the output code will be 0x7FFFFF.

I suggest measuring the voltage on pin 3, AINP1, to verify the actual input voltage to the ADC. With 1.25V input on pin 3, AINP1, the output code should be very close to 0x400000.

If you verify 1.25V on the input pin 3 of the ADC, and your output code is still saturated to 0x7FFFFF, then you may have a problem with your firmware. In this case, use a logic analyzer or oscilloscope and capture the /DRDY, SCLK, and DOUT signals over a full frame to confirm the correct data directly from the ADC output.

Also, for robust operation of ADS1278, remove the ferrite between the analog and digital grounds and provide a low impedance short. Ideally, the AGND and DGND are connected to the same ground plane.