Icl 7650 Chopper Input offset problem - Page 1
Forum section: electronics / beginners

author: Simon123
author_rank: Regular Contributor
timestamp: March 28, 2018, 04:03:10 pm
content: Hy!I built a simple circuit with this ICL7650SCPD amplifier. I used internal clock.I chose non inverting confirguration with 10R and 10k resistor, giving me gain of about 1000.Then I connected non inverting input to GND, and measured the output which was like 100mV.So this means input offset of 100uV(it should be between 1-5uV). What could be the problem? I assume the problem occurs because i wired everything on breadboard or is there something else?GND I was using is virtual ground by the way.

author: BrianHG
author_rank: Super Contributor
timestamp: March 28, 2018, 04:14:36 pm
content: What's you V+ voltage and V- voltage?

author: Simon123
author_rank: Regular Contributor
timestamp: March 28, 2018, 04:21:12 pm
content: +5V and -5V (From 10V total).Edit:Ok, i forgot bias resistor on non inverting input(watched daves video).Now i get like 10mV, that is 10uV offset. I assume i'd get it down to couple uV if i built it on PCB and used guard rings, right?Ol. now measured it again and its 50mV, 50uV offset.

author: Simon123
author_rank: Regular Contributor
timestamp: March 28, 2018, 07:42:02 pm
content: Changed setup to 220R/220k and measured voltage across both 220R resistors. I get about 0.5mV on each, and this means current of 2.3uA, WTF? Why do i have so much current flowing to the inputs :s ?

author: BrianHG
author_rank: Super Contributor
timestamp: March 28, 2018, 07:50:59 pm
content: Quote from: Simon123 on March 28, 2018, 07:42:02 pmNow, I am assuming you didn't buy your ICL7650SCPD from Ebay, Aliexpress, Alibaba, Banggood, or any other no name place, correct?Also, you have taken full anti-static precautions?Also, you are using a quality bread-board which doesn't have current leakage between adjacent pins...

author: Simon123
author_rank: Regular Contributor
timestamp: March 28, 2018, 07:54:20 pm
content: I measured voltage across that again and now its 0mV, but i still have offset from 20-50uV.No, i bought the chip from local electronics store.And im just using regular breadboard for 5€. Its not from ebay, i think its ok quality.And now i am getting negative output voltages (-33mV atm) ...Ill make circuit up on veroboard, i hope it removes this offset.

author: BrianHG
author_rank: Super Contributor
timestamp: March 28, 2018, 07:59:20 pm
content: Try moving the chip to a fresh new location on your bread board.Also, measure individual pins to check they are getting all the right voltages.Also, make sure you have 2 power decoupling caps right on the power pins to GND.Also, maybe show us a simple schematic of your wiring.Also, what evidence do you have that your IC isn't a Chinese clone.And, lastly, what kind and value of caps are you using for cextA and cextB.

author: Simon123
author_rank: Regular Contributor
timestamp: March 28, 2018, 08:20:06 pm
content: Polyester capacitors.I moved resistors from one hole to other and now i get -12uV. I guess its the board.

author: Simon123
author_rank: Regular Contributor
timestamp: March 28, 2018, 09:19:12 pm
content: Ok built everything on veroboard(with ic socket) and i get like 2mV output, which is 2uVSince 7650 has same pinout as 8 pin dip chips now i can also test offset of those.

author: BrianHG
author_rank: Super Contributor
timestamp: March 29, 2018, 01:28:26 am
content: Also add a .1uf cap between v+ and v-.And add a .1uf cap between GND and in the middle of the 2x 56ohm resistors.Things might get a tad better.

author: Simon123
author_rank: Regular Contributor
timestamp: March 29, 2018, 11:48:58 am
content: Thanks, i added capacitors across divider, hard to tell if there is any change, but offset is within spec anyway.