Comparator output with negative voltage - Page 1
Forum section: electronics / beginners

author: Ionforbes
author_rank: Contributor
timestamp: October 28, 2020, 08:13:56 pm
content: I'm working on a low resistance meter which uses4-Wire measurement. The input goes into an AD8230 differential amp with a gain of 10. Since it has to amplify voltages as low as 100uV for a 1mΩ rsolution, I had to use a dual-supply (I'm using +/- 5V) amp so input/output rail closeness isn't an issue. However the ADC I want to use, and all the ones I can find even, work on a single supply (+5V for me) so I can't let the differential amp output a negative voltaagee belw GND and fry the ESD diodes in the ADC.My solution was to buffer the output, use a jellybean LM311 comparator to short the buffered output to GND when the amp's output is negative and buffer again to protect the ADC input. I'm not sure which way to connect the collector and emitter of the output's BJT; collector to to GND and emitter to the buffer output or vice versa? My gut says connect the first one but then is that going to pull the negative amp outputupto GND or will it pull GNDdownto the negative amp output?Note that on the schematic images it should say 'ADC' not 'DAC' and the TSV912 is running on +/- 5VMany thanks,Ion

author: julian1
author_rank: Frequent Contributor
timestamp: October 28, 2020, 09:07:59 pm
content: An ideal diode/precision rectifier to clamp output voltage, might be another option.https://en.wikipedia.org/wiki/Precision_rectifier

author: Ionforbes
author_rank: Contributor
timestamp: October 28, 2020, 10:32:07 pm
content: That looks like a much better solution, but I don't understaand this bit :'The actual threshold of the super diode is very close to zero, but is not zero. It equals the actual threshold of the diode, divided by the gain of the operational amplifier'Does that refer the open loop gain of the op amp?

author: julian1
author_rank: Frequent Contributor
timestamp: October 28, 2020, 10:47:07 pm
content: Yes the open loop gain is for the op-amp.But I am actually not sure what the threshold of the diode is - perhaps the voltage related to leakage when reverse biased. I *think* that the op-amp input offset voltage might allow headroom - eg to measure down to a micro volt level difference from gnd.

author: tszaboo
author_rank: Super Contributor
timestamp: October 28, 2020, 10:53:08 pm
content: You are supposed to connect the REF pin of the AD8230 to a voltage higher than 0V. Say, connect it to 100mV. Low impedance. And then buffer this voltage with a single supply opamp. Camp the voltage between the two amplifiers with like a 1K resistor, diodes to the rails.

author: Ionforbes
author_rank: Contributor
timestamp: October 28, 2020, 11:35:51 pm
content: The AD8230 has multiple Vref pins and you need to offset all of them as shown in the datasheet, which is a pain to do. I'll go for the precision rectifier. I did a bit of research and there is eveen a full wave precision rectifier so i can read the negative voltage with a regular ADC and thus put the test leads in any orientation.

author: MasterT
author_rank: Frequent Contributor
timestamp: October 28, 2020, 11:50:41 pm
content: Simple solution would be to use 5V OPA with rail-to-rail inputs/ outputs. AD8230 has minimum +-4V (8 total), and is not the best choice. Even LM358 will e good enough, with offset correction in software.Here is an example: