Author Topic: Icl 7650 Chopper Input offset problem  (Read 730 times)

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Offline Simon123

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Icl 7650 Chopper Input offset problem
« on: March 28, 2018, 04:03:10 pm »
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.
 

Online BrianHG

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Re: Icl 7650 Chopper Input offset problem
« Reply #1 on: March 28, 2018, 04:14:36 pm »
What's you V+ voltage and V- voltage?
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Offline Simon123

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Re: Icl 7650 Chopper Input offset problem
« Reply #2 on: March 28, 2018, 04:21:12 pm »
+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.
« Last Edit: March 28, 2018, 05:35:17 pm by Simon123 »
 

Offline Simon123

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Re: Icl 7650 Chopper Input offset problem
« Reply #3 on: March 28, 2018, 07:42:02 pm »
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 ?
 

Online BrianHG

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Re: Icl 7650 Chopper Input offset problem
« Reply #4 on: March 28, 2018, 07:50:59 pm »
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 ?
Now, 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...
« Last Edit: March 28, 2018, 07:53:52 pm by BrianHG »
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Offline Simon123

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Re: Icl 7650 Chopper Input offset problem
« Reply #5 on: March 28, 2018, 07:54:20 pm »
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.
« Last Edit: March 28, 2018, 07:58:09 pm by Simon123 »
 

Online BrianHG

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Re: Icl 7650 Chopper Input offset problem
« Reply #6 on: March 28, 2018, 07:59:20 pm »
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.
« Last Edit: March 28, 2018, 08:01:02 pm by BrianHG »
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Offline Simon123

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Re: Icl 7650 Chopper Input offset problem
« Reply #7 on: March 28, 2018, 08:20:06 pm »
Polyester capacitors.
I moved resistors from one hole to other and now i get -12uV. I guess its the board.
 

Offline Simon123

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Re: Icl 7650 Chopper Input offset problem
« Reply #8 on: March 28, 2018, 09:19:12 pm »
Ok built everything on veroboard(with ic socket) and i get like 2mV output, which is 2uV :)
Since 7650 has same pinout as 8 pin dip chips now i can also test offset of those.
 

Online BrianHG

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Re: Icl 7650 Chopper Input offset problem
« Reply #9 on: March 29, 2018, 01:28:26 am »
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.
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Offline Simon123

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Re: Icl 7650 Chopper Input offset problem
« Reply #10 on: March 29, 2018, 11:48:58 am »
Thanks, i  added capacitors across divider, hard to tell if there is any change, but offset is within spec anyway.
 


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