AD7999
Production
The AD7991 / AD7995 / AD7999 are 12-/10-/8-bit, low power, successive approximation ADCs with an I2C®-compatible interface. Each part operates from a single...
Datasheet
AD7999 on Analog.com
Hi
I'm looking for some direction on how to go forward. My project needs to sample between 4 and 6 load cells/strain guages (wheat stone bridge with 4 wires: 2x excitation and 2x signal) with a sensitivity of 2mV/V at 1000 to 6000 samples per second. We have after numerous trials settled on the AD7999 for it's simultaneous sampling capability. My expertise is in the digital side of things, so I have so far managed to control and read the AD7999 eval board without issue. My problem is that the other ADCs we tested all had PGA gain ratios that could meaningfully measure the loadcell's 6.6mV differential signal (e.g. excitation voltage of 3.3V and PGA gain set to 128).
The AD7999 PGA gain only goes up to 8x which means that the maximum voltage output from the load cell comes nowhere close to vREF.
I need some advice on how to continue. Ideally we would like to stick to positive supply voltages where possible for all ICs and loadcell excitation but if it would really make things easier, it can be changed, especially with some guidance in terms of applicable part numbers.
1. Assuming only positive supply voltages, which instrumentation amplifier would be best suited apply the necessary gain, and how would the common-mode voltage of the bridge then be handled?
2. Would there be significant advantages to rather using bipolar supply for excitation and references?
3. Out of interest, what would happen if I were to narrow the vREF+ and vREF- very closely around the common-mode voltage (i.e. the unloaded signal voltage of the load cell). e.g. Assuming excitation is 3.3v and GND, what would happen if I were to set vREF- to 1.6V and vREF+ to 1.7V?
4. I found this module with a nice schematic and explanation that takes a differential sensor with small voltage output such a loadcell and converts it to a single-ended output with sufficient gain and bias compensation. protosupplies.com/.../ad620-instrumentation-amplifier-module/ . Can anyone advise whether this design is still modern or is there perhaps now a better solution in a smaller footprint?
