Is there a single chip ADC designed for reading a capacitor bridge circuit? - Page 1
Forum section: electronics / projects

author: ballen
author_rank: Regular Contributor
timestamp: August 13, 2021, 07:55:42 am
content: The HX711 is a "load cell amplifier". This is chip containing a differential 24-bit ADC, clock, and voltage regulators, for reading a resistive bridge circuit. They are used to read the load cell (strain gauge bridge) in inexpensive digital scales. Typical full-scale sensitivity is ~20mV, meaning that the single bit resolution is around 1nV.Is there a similar chip, intended to read out a capacitor bridge circuit?

author: Kleinstein
author_rank: Super Contributor
timestamp: August 13, 2021, 08:50:01 am
content: There are ADCs to read capacitors, though I don't remember the method they use. It may be a little different from a bridge.AFAIK Analog has some. Even some µCs (PIC) include HW for capacitance reading.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 13, 2021, 11:31:15 am
content: The FDC1004 from TI may well work for you. Also see AD5941.

author: ballen
author_rank: Regular Contributor
timestamp: August 13, 2021, 04:18:44 pm
content: FDC1004 looks good, thank you! Another one I just found is the AD7745.

author: David Hess
author_rank: Super Contributor
timestamp: August 13, 2021, 04:41:26 pm
content: Quote from: ballen on August 13, 2021, 07:55:42 amSingle bit resolution (quantitation noise) has nothing to do with true resolution when the input noise is 250 to 500 times greater. Lop off the 8 or 9 least significant bits.With 10 volts of excitation, that could deliver 80,000 counts but the typical application circuit only has an excitation of 2.5 volts limiting the resolution to 20,000 counts or 14.3 bits, which may seems low but is consistent with the accuracy of an excellent strain gauge load cell.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 13, 2021, 09:30:50 pm
content: FYI, I designed a nice bridge circuit (it's not a one-chip solution) specifically for small capacitances at a low frequency; I could ask the client to see if they're interested in sharing/licensing it. (Probably a long shot, but eh.)Tim

author: Terry Bites
author_rank: Super Contributor
timestamp: August 14, 2021, 01:42:28 pm
content: Yes the AD7745/AD7746 look like the very thing you want. I wasnt aware of them, thanks.

author: David Hess
author_rank: Super Contributor
timestamp: August 14, 2021, 06:57:31 pm
content: Also take a look at how the signal conditioning of relative humidity sensors works. They are capacitive with a range of about 350 to 550 picofarads but the concept works over any range.

author: spostma
author_rank: Regular Contributor
timestamp: September 14, 2021, 07:44:31 pm
content: some microcontrollers have ADCs with capacitive voltage divider stages.Microchip calls this CVD ADC, intended for capacitive touch sensors.

author: babysitter
author_rank: Frequent Contributor
timestamp: September 16, 2021, 01:43:59 am
content: Have a look at this:https://www.pmt-fl.com/picocap/picocap-capacitance-measurementExample datasheet (aged?) here:https://www.sciosense.com/wp-content/uploads/documents/Datasheet-PCAP02-Volume-1.pdfBRHendrik