PGA/AFE with input offset correction DAC? - Page 1
Forum section: electronics / projects

author: PioB
author_rank: Regular Contributor
timestamp: November 24, 2023, 11:05:54 am
content: I am looking for a special PGA / AFE with offset correction DAC to amplify (and in the end digitize) whetstone bridge sensors. The problem is that they are screen printed and thus not all four resistors are the same size leading to (relatively) large offsets from the common mode voltage (VCC/2) of the two differential paths.(offset up to +-300 mV for VCC=3.3 V)The absolute signal changes are small (some mV) so a regular PGA of 4x or 8x leads to (one of) the signals being dangerously close to the /at the rail, which leads to an output of 0 or 2^24, depending on the precision of the printThere used to be a Exar/maxlinear/resurgent semiconductor chip, XR10910 which could connect up to 16 whetstone bridges and correct the offsets with a DAC before amplifying the signals.https://resurgentsemi.com/wp-content/uploads/2020/02/XR10910_DS_1E_011519_R.pdf?d453f8&d453f8Now the only similar chip I was able to find is a MCP3911 which is only two channelhttps://ww1.microchip.com/downloads/en/DeviceDoc/MCP3911-3.3V-Two-Channel-Analog-Front-End-DS20002286D.pdfthe offset compensation seems to be in the digital domain after the ADC, then a digital amplification is performed (bit-shift?)From the data sheet:"DATA_CHn(post-cal)=DATA_CHn(pre-cal)+OFFCAL_CHn)*(1+GAINCAL_CHn) (from data-sheet)The gain calibration adds 1x to this register and multiplies it to the output code of the channel, bit-by-bit, after offset calibration."Unfortunately they don't seem to have a demo board available anymore on digikey/mouser for a quick test.If someone has an idea for an amplifier/afe, ideally with 4-8 differential channels that would perform the offset-correction I'd be grateful for any hints.

author: colorado.rob
author_rank: Frequent Contributor
timestamp: November 24, 2023, 12:25:02 pm
content: STM32L4+ microcontrollers have two 12 bit DAC channels, opamps that can operate as PGAs, each of which can be fed directly into a 12 bit ADC. I use the DACs for input compensation to the PGA. Works well. You would just need to add a couple analog muxes before the input of the PGAs.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 24, 2023, 06:52:12 pm
content: The offset correction with a DAC is tricky: the DAC also needs to be very low noise to make sense. One would still need to cover the full dynamic range as the the extended range without the offset correction.So the µC internal DAC would not be good enough.In most cases one would today opt. for an ADC with better resolution / lower noise so that the range is larger and the offset can be corrected afterwards. SD ADCs got quite precise and low noise.A possible work around may be 1 selected resistor in parallel to 1 bridge resistor to reduce the offset somewhat (e.g. 5 x) and then use less extra ADC resolution.

author: PioB
author_rank: Regular Contributor
timestamp: November 27, 2023, 09:27:46 am
content: Thank you very much for the idea with the MCU with integrated DAC and amplifiers, I will check that out. The resistors in parallel is a bit non-practical as the sensors get soldered onto the PCBs and it'd be probably a bit much characterizing them all individually post install and adding resistors to bring the resistance up.I found another PGA for correcting the output offset of the sensors, but it's "a bit" expensive:https://www.analog.com/en/products/ad8338.html

author: voltsandjolts
author_rank: Unknown
timestamp: November 27, 2023, 09:46:44 am
content: Quote from: PioB on November 27, 2023, 09:27:46 amThis might be easier than you think. Since you are just looking to remove most of the residual offset, you don't need to apply known force/pressure/whatever to the sensors. So the setup is easy. Then just do a little bit of custom software to read the offsets, calculate required resistance and tell the assembly tech what resistor values to solder in what locations, to correct the offset. I've done this in the past and built that into the instrument firmware.If you don't use the soldered resistor option, I think your one-chip solutions are limited, not many PGAs have offset correction for such high residual offset. I've used TIPGA308in the past with good results, but it can't correct your offset. TI might have something else that can fit your requirements.

author: PioB
author_rank: Regular Contributor
timestamp: December 01, 2023, 01:51:12 pm
content: While browsing the TI site, I found a document,https://www.ti.com/lit/an/sbaa077a/sbaa077a.pdf, mentioning several ADCs that have an offset correction DAC stage before the PGA, namely the "ADS1216, ADS1217, ADS1218, ADS1240, ADS1241, ADS1242, ADS1243, MSC1210"Would be nice, if they brought this feature to more modern ADCs too, the ADS1243 document seems to originate in 2001 (with revisions)Also the PGA302 might be worth a look, but is on ly for one channel and not available on digikey right now.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2023, 04:52:23 pm
content: Finding suitable resistor values can be relatively easy, especially if the trim does not have to be that accurate. Modern ADC usually have enough resolution to get away with quite some offset. So limiting factor is often more the DMS itself and the amplifier, not so much the ADC. So use less gain from the amplifier and a little more ADC resolution.With the CMOS processes used for the SD ADC chips it is a bit tricky to implement a low noise DAC. A noisy DAC would only allow correcting a small offset (e.g. 1 % of the range) that the main ADC could as well handle with a little more range.