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I'm looking to use the ADS1232 to measure inputs from two full-bridge pressure sensors. I'm just a little confused about the function of the A0 pin and the filter settling time.
1. In the ADS1232 (not the ADS1234) does changing A0 select the differential input pair (e.g. low = AINP1/AINN1, high = AINP2/AINN2) or does it have a different function? This isn't entirely clear in the data sheet, which seems to me to say that only AINP is changed by changing A0.
2. If I want to measure from two full-bridge sensors 'simultaneously' (really interlaced) with SPEED=0 (10SPS), do I change A0, discard 5 readings and take the 6th (settled filter), then change A0 again, discard 5 readings and take the 6th etc... Won't my effective sampling rate per channel drop to 2SPS or worse?
I've attached a screenshot of my EAGLE circuit so you can get an idea of what I'm talking about. Note that I've tied A0 to DGND by mistake.
Many thanks for your help,
For question 1, the A0 pin selects the input pair of the differential input. For question 2, the multiplexing does reduce the actual throughput of your measurement. The latency is based on the digital filter which needs multiple samples before the digital filter fully settles. You could change the SPEED pin setting to 80sps, but this will increase the noise. You may also want to add some analog input filtering to reduce external noise (EMI/RFI) and for anti-aliasing.
I don't know your total analog current requirements, but keep in mind that some bridge sensors will require more current than the REF5050 can provide. You may want to use a low noise LDO instead of the REF5050.
I don't know your expectations, but if you need faster response with 50/60 Hz filtering you might want to take a look at the ADS1247 or ADS1220. These two devices will require writing configuration information to internal registers (unlike pin control of the ADS1232) but they offer 50/60 rejection at 20sps and are true single-cycle settling (no data needs to be thrown away).
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In reply to Bob Benjamin:
Thanks very much for your quick reply Bob!
I'll certainly look at those other two chips you suggest. I may even find it easier to use a standard SPI protocol than the modified 2-wire system of the ADS1232.
As for the REF5050, I used it previously to act as the supply in a design with one of these bridge sensors and an ADS1100 as the AFE and it worked very well. I'm hoping to pull off the same trick here with two of the sensors.
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