ADS131M08: Data converters forum

Jessica,

A good choice of the ADC depends on the functions and characteristics that are important to your design. For example, what data rate and resolution are you looking for in your design? Is simultaneous sampling required for your application? Or can you use a multiplexer with one ADC. What input range do you intend to measure? What reference do you intend on using?

If you intend on using a Wheatstone bridge type measurement, many use the supply to drive the bridge and the reference, so that the measurement is ratiometric. Because of this, the ADS131M08 may not be the best choice because the reference input is limited to 1.3V max.

Before I suggest another ADC, it might be best for you describe the system you're working on and what ADC characteristics are important to your design.

Joseph Wu

 Thank you for your immediate response. I write the desired system here.

update rate: read all 8 ADC channels about 1 ms

simultaneous sampling is better but it's not a limit

effective ADC resolution: 1uV

max input voltage: excitation voltage * 4.5 mV

max excitation voltage: 10V

we are not limited to power consumption

we want to design our system like this:

Jessica,

The big problem that I see right now is how you drive the bridge. The ADC inputs are limited to AGND-1.3V on the low end to AVDD on the high end. The inputs are limited even further if the PGA is used in gains higher than 8. Because you have such a small input signal, you'll likely need the gain. With the PGA greater than 8, the upper limit is limited to AVDD-1.8V. This means that the analog input could only be as high as 1.5V. If the bridge is driven by a 3.3V supply, your outputs would be at 1.65V. If your bridge is driven by a 10V supply, then the outputs of the bridge would be near 5V. That would be far outside the input range of the device. To use this device with that large of a bridge excitation directly, you couldn't use that large of a unipolar supply. If you want to drive the input with a large voltage, you would need a bipolar supply.

Alternately, you could use an instrumentation amplifier at the output of the bridge, this could be used to both give some amplification, and to level shift the signal to the input range of the ADC. Note that adding any intermediate stage will add error. There will be some amount of gain error, offset, and noise added to the measurement.

So for your system, can you provide even more detail? What characteristics will be important to you? What supplies will you have in your system?

Joseph Wu