ADS1248 setup

Hi Marc,

1. Setting the start pin high will configure the device to read data continuously. 

The DOUT/DRDY (pin26) has two modes:  data out only (DOUT), or data out (DOUT) combined with DRDY.  The DRDY MODE bit determines the function of this pin.  The ADS1247/48 detailed register definitions are described on page 43.   

a)      When the DRDY MODE bit is set to ‘0’ (default) this pin functions as a DOUT only.  The conversion result can be received once from the device after the DRDY signal (pin 25) goes low after sending 24 SCLKs.  You must ensure that the complete data retrieval (register read-back) before DRDY goes low, or the resulting data will be corrupt.  Page 36 on the datasheet talks about this mode.  The CS pin is low during this operation.

b)       When the DRDY MODE bit is set to ‘1’ the DOUT/DRDY (pin 26) functions both as DOUT/DRDY.  This combined pin allows for the same control with fewer pins.  Page 36 describes this mode of operation.  The CS pin is low during this operation.

2. The internal reference of the ADS1248 is 2.048V.  Any differential voltage above 2.048V is going to read full scale positive 7FFFFFh. When selecting your reference voltage you should consider your full scale input voltage, and the gain setting of the PGA. If your PGA gain is set to a Gain=1; and your expected maximum differential voltage is 3.3V; you should use an external reference with at least 3.3V, and should not exceed (AVDD-AVSS)-1, so you meet the Full-Scale Input voltage requirement as described on page 4.   Please refer to page 4 and page 25 on the datasheet for the Reference input requirements.  Also please consider the common-mode requirements in your design.

3. Please connect any unused analog pins to GND; outside of the analog pins which pins do you have floating or do you have specific questions?

Thank you and Regards,

Luis Chioye

Hi Luis

Thanks so much for your quick reply, as always I have found the support on this site to be excellent.  I have read the sections you pointed to in your reply and think I have it all sorted, if its not asking too much I'll put my pinouts up and if you can just give me a quick indication if there's anything massively wrong.  The only point I'm still slightly hazy on is the Vref.  Both sensors run off +3.3V with a zero'd output off VDD/2 or 1.65V, I'm assuming then that I'll have VrefP0 as 3.3V and just put VrefN0 to ground.

Anyway here's my pin description.

Thanks again for all your help I know some of this probably seems elementary but  this is the last part of my design and I just want to double check everything before I get something built.

Warm Regards

Marc

Hi Marc,

I believe most pin connections seem to be ok, you may want to connect pins REFP1(pin 7) and REFN1(pin 8) to GND if you don't plan to use a second external reference.

I have a couple of questions regarding the sensors. You mention that the sensors run out of 3.3V with a zeroed output of 1.65V.   What is the maximum and minimum differential voltage (VIN= ADCINP-ADCINN) that you expect at the inputs of the ADS1248? Is it possible to obtain a schematic that shows how the inputs are connected to your sensors? This information will help decide the reference voltage needed, optimal PGA gain setting and to verify that we meet the common-mode input voltage range requirements.

Thank you and Kind Regards,

Luis

Hi Luis

Again, thanks for all your help, I'm unable to get you the data today and we have public holiday tomorrow so I'll get it to you on first thing on Thursday.  I will also forward you a copy of the schematic, is pdf fine?  Also did you want me to post it up here on the forum or email to you privately?

Thanks again.

Marc

Hi Luis

I've attached a copy of the schematic so far, its incomplete but should give you an indication of the proposed connectivity  to the ADC, all anaolg sensors are on the same sheet as the ADC. 

As far as the sensor voltage particulars, the Accelerometer has a zero output of 1.65V with variances of +- 660mV/g up to a max of 2g, so absolute max of 2.97V and min of 0.33V.  We will not come close to these values in any of our proposed applications and in most cases will see variances of only +-10mV.

The Gyro has a zero level of 1.23V and variances of 33.3mV/degree/s up to 30degree/s so max of 2.22V and min of 0.24V.  As with the accel our operational range will be far smaller than the maximums.

Not sure whether it is relevant or not but this sensor board will interface with a TI OMAP3530 as part of a Gumstix SOM. 

Again thank you for all your assistance.

Warm regards

Marc

3225.sensors_vreg.pdf

Hi Luis

I hopefully just have a quick couple of clarifications and then I will leave you alone to help the rest of the people on this forum.

1. I have changed the AVDD to 3.3V at your suggestion and read up on setting the VBIAS, am I setting a spare pin, all pins or connecting externally VREFN?  All inputs are single ended so I'm just a little confused with the P/N

2. Same with the generated V from the IDAC, is this taken as an external VREFN or all internal, also if VREFN is VREFP just AVDD?  I have a Vref of 1.23V available from the sensor, will it be easier to use this or by using the IDAC.

3. If everything above is sorted internally should all left over pins be tied to GND?

Once again thanks for your help, I hope this process hasn't been to painfull for you.

Cheers

Marc

Hi Marc,

Please find answers to your questions below.

1) It was mentioned on the previous post that the accelerometer has a zero output of 1.65V, with output up to 2.97V and min 0.33V. You may set up the inputs of the  ADS1248 single ended or differential. 

a) Differential ended. Connect AVDD=3.3V. Use the internal voltage reference of 2.048V and reserve a spare channel for the IIN input to connect to the VBIAS function of 1.65V and the positive inputs (INP) to your sensors outputs.  In this set up your digital output would be close to the ideal 000000h when your sensor is zero (1.65V).  The ADS1248 will sense a differential voltage in the range of +/-1.3V; and yawill use the internal 2.048V reference.  The VREFP0 and VREFN0 should be grounded unless you have decided to use an external reference.  When using the internal reference, the VREFCOM should be connected to GND, VREFOUT should have a capacitor (see page 29 on datasheet).

b) Single Ended. The user will need to connect AVDD to 5V and use an external Reference that exceed the maximum expected differential voltage (2.97V).  VREFP0 and VREFN0 would be connected to the external reference (VREF and GND).  The positive inputs (INP) need to be connected to the outputs of the sensor and INN will need to be connected to GND.  The user will need to take to account in software which codes correspond to zero, max and min; since the signal seen by the ADS1248 ranges from +0.33V to 2.97V. 

NOTE:  Please review the common-mode input range for the minimum signal  when using the single ended input setup (i.e. when your sensor is at 0.33V you may run out of common-mode range, but if the user does not expect to reach this condition, it may not be an issue).   

2) If the gyro sensor has a zero level of 1.23V; and since the sensor provides a Vref of 1.23V,  the user will probably connect the negative input (INN) to the Gyro's 1.23V Reference; and the positive inputs (INP) to your sensors outputs. Using this set up, the ADS1248 digital output will be close to the ideal 000000h when the sensor is at the zero level.  For this sensor, the user could plug in your VREFP1 to the Gyro's 1.23V and your VREFN1 to GND.  Using this set up, the user could select with software if he wishes to use the Gyro's reference 1.23V (external reference) or the internal 2.048V reference. 

3) Analog inputs that are not used/reserved for INN, INP signals should be grounded.  If the external reference inputs are not used, leave them grounded.

NOTE:  The ADS1248 can only monitor one channel at a time and the device needs to change channels to monitor the different sensors; the user must allow time to change channels and re-configure the device to monitor the different sensors. 

Let me know how your project turns out.

Thank you and Regards,

Luis

Luis

That pretty much wraps it up, sorry for all the back and forth just wanted to make sure I had it spot on before sending off for prototyping. Fingers crossed, I'll let you know how it all plays out.

Thanks

Marc