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Sigma delta ADC question, PGA280 and ADS1271

Bonnie Baker - WEBENCH Design Center
Expert 3555 points
Other Parts Discussed in Thread: ADS1259, PGA280, ADS1271

Sanjeev,

 

I agree that an extra amplifier it may be a bit much and maybe unnecessary. We are developing a circuit for the PGA280 and the ADS1259. A cut-out of the PGA/ADS1259 combination is below. This circuit will appear on the EVM, when it is released. We are using the recommended PGA280 filter for the ADS1259 EVM.

 

 

There are a few issues to consider when you interface these two devices. The first issue is that the PGA has chopper amplifiers inside and it is important to reduce the output chopper noise at its output. Hence the recommended filter in the data sheet. This filter have been very carefully designed to reduce the chopper noise and maintain PGA208 stability throughout gain changes. The second issue is the corner frequency for the ADS1271. As you might notice, the ADS1271 high frequency speed is approximately 10 times higher that the ADS1259. The corner frequency difference in the PGA filter and the ADS1271 filter is also about 10x, where the ADS1271 filter is 10 times higher. I am not pointing this out to motivate you to use the higher cut-off filter between the ADS1271 and PGA208, rather I want you to know that you need to use the ADS1271 in a high resolution or low power mode. I am hoping that your input signal bandwith will allow this.

 

 

Best Regards,

 

Bonnie Baker

 

 

-----------------------------------------------------------------------------------------------------------

From: Sanjeev 
Subject: Sigma delta ADC question

 

Dear Ms. Baker,

 

After following several articles that you have written on sigma-delta ADCs

and general advice about dos and don'ts, I wonder if you could comment on a

issue that we are having with a new TI part. I have tried to discuss this

matter with your technical support department, but to be honest I get the

feeling that they are really sure either.

 

We are trying to connect a PGA280 to an ADS1271 for our application.

However, the issue regarding the anti-aliasing filter is something that has

led to an certain amount of confusion. According to the PGA280 datasheet, a

simple two resistor/one capacitor arrangement connected from the PGA's

output to the ADC's input should work. However, according to the ADS1271's

datasheet and ADS1271EV board, the two resistors are enclosed inside the

driving op-amp's closed loop. Using this arrangement, there is a very low

output impedance driving the ADC/capacitors. As I believe an extra buffer

op-amp is over the top, my question is simply, will the simple two

resistor/one capacitor (from the PGA280) approach work well with the

ADS1271?

 

With kind regards,

 

Sanjeev

 

 

  • Prodigy 10 points

    Bonnie,

    I read with much interest your response to Sanjeev.  But I am confused about part that says that the PGA280 filter corner is lower than that of the ADS1271.  Are you referring to the output filter?  I see in the data sheet that the recommended output filter is 100 Ohms in series with 10nF, which I calculate to have a corner at 159kHz.  This is quite a bit higher than the corner of the ADS1271 FIR filter (42.2kHz in high speed mode).  What am I missing here?

    Bill

  • in reply to Bill_H
    TI__Expert 3555 points

    Bill,

    Actually the modulator is runner much faster than 42.2kHz. Of course this depends on your clock, but at the maximum fclk frequency you would see the modulater running around 5-10 MHz. This is one sampling conversion that occurs in the converter and the output data rate is a second. For the ADS1271 running in its high speed mode the output data rate is 105 ksps.

    In this discussion I am talking about the filter between the PGA280 and the ADS1271. For the PGA280 the output filter (for that device) is a recommended 100 ohms and 10 nF. This filter attenuates the switching noise caused by the internalchopper stages. You are right. The corner frequency of this fitler is 159 kHz. This corner frequency is much too low for the bandwidth of the ADS1271. The output data rate of this device is 105 ksps so you can imagine that signals near this higher frequency would be attenuated by this 159 kHz filter. Keep in mind that the ADS1271 24-bit converter.

    Bonnie Baker