Interfacing of PGA280 with ADS1259

Please reference the ADS1259EVM user guide @ http://www.ti.com/lit/ug/sbau163a/sbau163a.pdf.  The schematic of the EVM (at the end of the document) should help answer some of your questions.

 Hi Chaganty,

Welcome to the TI E2E forums!

1) Typically, the PGA280 output supply pins (VSOP & VSON) are shared with the dataconverter supply to reduce the design complexity. How do you plan on configuring the the ADS1295 supply; is this supply shared? A supply of +/- 2.5V would be a standard value; however, I see that you are correctly considering the PGA output swing by inceasing the

a) I have two comments on this question:

• The PGA output will swing to within +/- 100mV of the the VSOP and VSON supplies. Therefore, with +/-2.7V supplies you would be able to support a +/- 2.5V output swing. However, the 20mV input with gain of 128V/V will actually go higher, (up to +/- 2.56V), which may exceed the input voltage range of the ADS1259 (+/- Vref). 
• Also, you need to consider the common-mode voltage of your sensor. Your sensor input to the PGA280 must somehow be referenced to VSP and VSN to not exceed the PGA's input common-mode voltage. How is your sensor connected to the PGA?

b) VOCM will set the output common-mode voltage of the PGA280. This can be connected to "REFOUT" on the ADS1259. "REFOUT" will be +2.5V above AVSS, which is mid-supply for a 5V supply (this holds true for uniupolar and bipolar supplies).

2) This depends on the types of supplies you are using for both the PGA280 and ADS1259. (Ignoring VSP and VSN for now)...typically,

• Unipolar supply
  if VSOP (PGA) = AVDD (ADS) = +5V
  & VSON (PGA) = AVSS (ADS) = 0V
  then VOCM (PGA) = REFOUT (ADS) = +2.5V

• Bipolar supply
  if VSOP (PGA) = AVDD (ADS) = +2.5V
  & VSON (PGA) = AVSS (ADS) = -2.5V
  then VOCM (PGA) = REFOUT (ADS) = 0V

3) This would depend on how you are genereating each supply. There is some flexibility in how each supply is referend to the others. VSN, VSON, and DGND would only be tied together if all your supplies are unipolar and referenced to the same ground. If this is the case, be careful how you apply a "star" ground!! You always must consider how returns currents will return back to their source. You want this to be a short/low impedance/non-loop forming path. It is best practice to have a PCB layer dedicated to a single ground plane (both analog and digital, unless isolation is required), which is "partitioned" (not "split", meaning there is no copper cutout)  into anlog and digital sections.

4) You've come to the right place to find answers. I hope you find this information most useful!

5) Note well taken! In addition to the link Greg referred you to, also look at page 41 in the ADS1259 datasheet for a PGA280/ADS1259 example.

Regards,
Chris

Hello Chris,
Many thanks for the suggestions. I am attaching (pdf file) indicating the simplified block schematic w.r.t interconnections between PGA & ADC. Request you to offer your suggestions. The answers to your questions/suggestions are:
1. My input may not cross 18.000 mv in actual practice. If neded I will place a passive attenator at output of PGA with 5 ppm resitors  to reduce the gain from 128 to 110
2. The sensor is a pressure sensor connected to a bridge and excited by isolated 10V supply. I am amplifying the output of bridge signal, converting it to digital, isolate the outputs, SPI, control signals through optoisolator and giving to microcontroller which has a different isolated supply. The input analog signal amplifier(PGA) and ADC are on a seperated card powered by isolated power supplies (derivd from 230V AC).Thus it is difficult to quantify the common mode voltage. System requirements indicate that the output of sensor could have 500V DC as common mode voltage w.r.t the microcontroller ground. Thus the common mode voltage definition is little vague as the whole analog amplifier+ADC card would get lifted (by 500V) and the optoisolator's leakge could cause error due to common mode voltage. I do not know how to caleculate this error. 
3. In the block schematic I have attached I am using two grounds one for analog and other for digital. I need to short this at some point, and I am thankful for your suggestions. I will consider ground return paths in my layout. I will use a  4 layer board, one layers dedidcated for analog and digital grounds.
Thanks once again,
Regards,
Chaganty

Hello Chaganty,

I did not get the attachment. Would you mind sending it again? If you prefer, you can click on my name and start a private conversation with me so that only I will be able to see the schematic.

Regarding the information in your last post, I have a few more comments...

1. I would try to get by without the passive attenuator on the PGA output if possible. You will gave some additional gain error and added noise (See this article writen by my colleague for as reference on resistor noise: http://www.edn.com/electronics-blogs/the-signal/4402494/Resistor-Noise-reviewing-basics--plus-a-Fun-Quiz). You most likely will get a more accurate result by measuring the input with a PGA gain of 64 (V/V).
   
   
2. Thanks for this information. I may have confused you about common-mode voltage. For now, just consider your PGA and ADC card and ignore the other isolated parts of the system. You have a differential input voltage of +/-18mV to +/-10V into the PGA (This is the potential difference between the positive and negative inputs). The common-mode voltage is the average voltage of the PGA inputs referenced to the PGA supply voltages (i.e. the common-mode voltage = [INP1 + INN1]/2. Your PGA may have +/-15V supplies referenced to some ground potential. You may then have a +10V input where INP1 = +5V; INN1 = -5V (common-mode voltage = 0V), or possibly you may have a +10V input with INP1 = +12.5; INN1 = +2.5V (common-mode voltage = +7.5V).
   
   You will have to consider the input ranges of the PGA and ADC. VOCM, sets the common-mode voltage of the PGA output/ADC input. However, your sensor must somehow be referenced to the PGA supplies; it cannot be a "floating" input when referenced to the PGA supplies.
   
   
3. I'll be glad to look over your schematic and give feedback.

Best regards,
Chris