AD623 in single power - output saturates

[krmt]

Hi there,
new to this community, I already need to ask for some help:

I'm working with the AD623B but i get somewhat strange results. The problem seems to be that I operate the omamp in single supply mode. Everything works fine but the output wont get any higher than 1.16V, independent from the opamp's gain. Operating the opamp with dual supplies, the output is as expected.

I attached a multisim screenhot. (ps: multisim produces a simulation error for an output higher than 1.16V. Using a breadbord, the output saturates at the same value...)

I really appreciate any help.
thanks,
krmt

 

[ericgibbs]

hi,

Try removing the 0V connection to pin2, lets know what you get.

Keep the 1.16 mV signal source connected.

 

[ericgibbs]

hi,
Look at this page from the AD623 data.

You are working below the allowed limits of the CMV.

Connect pin5 to a +2V supply, use a divider resistor chain from +5V
Reconnect the 0V to pin #2.

OK.?

 

[krmt]

Hi ericgibbs,
thanks for your really prompt answers.

i disconnected the ground from pin2 - the simulation isn't running anymore...

am i working below the limit? pin 5, however, just set a offset voltage as reference to the output. I thought, since i grounded my input signal it should be fine to have a reference of 0V, too.

Connect pin5 to a +2V supply, use a divider resistor chain from +5V

what do you mean by that - what for do i need the voltage divider?

maybe you can explain your idea a bit more in depth?
thanks again,
krmt

 

[Roff]

Hi there,
new to this community, I already need to ask for some help:

I'm working with the AD623B but i get somewhat strange results. The problem seems to be that I operate the omamp in single supply mode. Everything works fine but the output wont get any higher than 1.16V, independent from the opamp's gain. Operating the opamp with dual supplies, the output is as expected.

I attached a multisim screenhot. (ps: multisim produces a simulation error for an output higher than 1.16V. Using a breadbord, the output saturates at the same value...)

I really appreciate any help.
thanks,
krmt

What do you mean, "independent from the opamp's gain"? Have you tried it with a gain over 1000? 1.16mV*1000=1.16V. That's what you are getting.

 

[Roff]

hi,
Look at this page from the AD623 data.

You are working below the allowed limits of the CMV.

Connect pin5 to a +2V supply, use a divider resistor chain from +5V
Reconnect the 0V to pin #2.

OK.?

I read it to say the CM range extends below GND. The datasheet also says that.

EDIT: I think I was incorrect. Fig. 22 in the datasheet appears to confirm what Eric said. Adding a common mode bias should help.

 

[audioguru]

It is an odd opamp. Its input common-mode range does extend below ground. But when it does then the output high voltage is reduced.

 

[Roff]

It is an odd opamp. Its input common-mode range does extend below ground. But when it does then the output high voltage is reduced.

I didn't see Fig. 22. I think Eric is correct.

 

[ericgibbs]

I didn't see Fig. 22. I think Eric is correct.

hi Ron,
Thats the way I read it.
At the moment I have a AD623 setup on my project board for a weigh scale job, as soon as I finished that I'll do some tests to check out the AD623 at very low levels of CMV etc.... let you know the results.

Regards

 

[ericgibbs]

what do you mean by that - what for do i need the voltage divider?

maybe you can explain your idea a bit more in depth?
thanks again,
krmt

hi,
Connect a 3K3 and 2K2 resistor is series and connect one end to +5V and the other end to 0V. The centre of the two resistors goes to pin 5, the REF pin.

This will raise the Vout, with no input signal to +2V, now retest using the 1.16mVdc source.

Lets us know the results/reading.

 

[ericgibbs]

Results

hi,
Configuration:
Gain SET to 500
pin2 to 0V,,,, pin3 connected to 0V via 4K7

With a :
Connected a variable voltage to input pin3 [via a 4k7]

Vsig of +1.2mV on pin3, the output pin6 reads +0.700V, Gain pot has correct effect on Vout level.
Vsig of +3.6mV on pin3, the output pin6 reads +1.160V at which point Vout starts to clamp
Vsig of +5.5mV the Vout reads +1.176V
and remains at this voltage even when Vsig is increased to any value over 5mV.

Disconnected pin5 REF from ground and connected to +2V... the Vout now clamps to+3.176V

Conclusion:
According to the AD623 datasheet with a zero CMV the AD623 can amplify very low level signals.
These simple tests suggest that the signal level must be between 0.00mV and + 3.5mV.

When a +2V offset is applied to pin5 REF, it moves the standing Vout to +2V, the small Vsig still applies
but now the Vsig can swing 0.0 thru +/-3.5mV.

The AD623 OPA is not a suitable choice for this low mV application..
A better choice would be the MCP6291/2 , get the datasheet from Datasheet Archive - Free Datasheet Search Engine :: PDF Datasheets :: Data Sheet :: Datasheet :: Application Note : çš„å￾‚æ•° : 规格书 : 技术资料 : ë￾°ì￾´í„°ì‹œíŠ¸ : データ・シート : Fiche technique : Datenblatt : опиÑ￾ание

 

[krmt]

Thank you so much everybody for this many answers!
I didnt notice figure 22 and 23 - thanks for the info.

@eric: what you posted with your last answer is exactly what i meant to say. Whether i have a gain of 100 or 1000 the output will not exceed 1.16V.

In my application, the input ranges from 10uV to 4mV, which i want to amplify by a factor of 1000 (resulting in an output range of 10mV...4V) Thus i cannot use the reference pin to apply an offset as the output would exceed the supply voltage... Do you see a solution or am i trying to beat physics?

I'm kind of lost choosing the right opamp for my needs. Can you give some advice!

ps: thanks for explaining the voltage divider - just wanted to know if i could use a +2V power supply to pin5 directly, instead

 

[Roff]

Thank you so much everybody for this many answers!
I didnt notice figure 22 and 23 - thanks for the info.

@eric: what you posted with your last answer is exactly what i meant to say. Whether i have a gain of 100 or 1000 the output will not exceed 1.16V.

In my application, the input ranges from 10uV to 4mV, which i want to amplify by a factor of 1000 (resulting in an output range of 10mV...4V) Thus i cannot use the reference pin to apply an offset as the output would exceed the supply voltage... Do you see a solution or am i trying to beat physics?

I'm kind of lost choosing the right opamp for my needs. Can you give some advice!

ps: thanks for explaining the voltage divider - just wanted to know if i could use a +2V power supply to pin5 directly, instead

According to Fig. 23, if you can add a common mode offset on the input of ≈1.8V, instead of connecting the inverting input directly to GND, you will get the maximum output swing.
Do you realize that the input offset voltage can be 200µV, which is 20 times your minimum input voltage? Can you correct that in software? Also, the input offset alone seems to mean that you will have to use a DC bias on the Ref pin, and subtract that voltage from your output measurement. -200µV * 1000 = -200mV. The output can't go below GND.
Have you considered using dual supplies?

 

[ericgibbs]

In my application, the input ranges from 10uV to 4mV, which i want to amplify by a factor of 1000 (resulting in an output range of 10mV...4V)

I'm kind of lost choosing the right opamp for my needs. Can you give some advice!

ps: thanks for explaining the voltage divider - just wanted to know if i could use a +2V power supply to pin5 directly, instead

Hi,
I would buy a MCP6291 single or MCP6292 dual OPA for < £1.0GBP rather than the AD623 which is three times the cost!.
Get the MCP datasheet from the link in my post.

Hi Ron,
I'll run some more tests tomorrow regarding the input pin2 not grounded and use an isolated mV source. Also setting 2V on the REF pin.

Let you know the result.

Hi Again Ron;
EDIT:
Relooked at fig23, I read it a different way, is there a beer [ or two ] riding on this.?

 

[Roff]

Hi,
I would buy a MCP6291 single or MCP6292 dual OPA for < £1.0GBP rather than the AD623 which is three times the cost!.
Get the MCP datasheet from the link in my post.

Hi Ron,
I'll run some more tests tomorrow regarding the input pin2 not grounded and use an isolated mV source. Also setting 2V on the REF pin.

Let you know the result.

It appears from the datasheet and from simulation that 2V on Ref will offset the output 2V above GND. Since his input never goes negative, that will waste 2V of his output swing. Perhaps something like 100mV would be better on Ref.

 

[ericgibbs]

It appears from the datasheet and from simulation that 2V on Ref will offset the output 2V above GND. Since his input never goes negative, that will waste 2V of his output swing. Perhaps something like 100mV would be better on Ref.

I edited my last post.
I've chosen 2Vref as an arbitary value for the tests.
As you say if it works with some level of offset, then a figure of 100mV makes sense.

 

[Roff]

EDIT:
Relooked at fig23, I read it a different way, is there a beer [ or two ] riding on this.?

Next time you're in Boise, look me up. I'll buy you a beer.
Or two.
The chances of me getting to the UK are slim to none.

 

[krmt]

Guys, your response is just awesome! That's a great welcome As you probably figured out, I am not the electronics guy, but i follow all your steps and try to get my application running!

Hi,
I would buy a MCP6291 single or MCP6292 dual OPA for < £1.0GBP rather than the AD623 which is three times the cost!.

. I'll have a look at that opamp. what makes it superior over my current 623?

In terms of the CM issue given through figure 22, i came across the brand new AD 8223, which with figure 20, page 11 does not seem to have this problem. Do you think, it could be suitable too. unfortunately, there is no model for multisim yet...

thousand thanks again,
/me

 

[ericgibbs]

Next time you're in Boise, look me up. I'll buy you a beer.
Or two.
The chances of me getting to the UK are slim to none.

Hi Ron,
I have run the tests in the last few minutes, couldnt go to bed not knowing.!!

With a 2Vref and a floating mV input it clamps at about 2.5V

I'll re-run the test in the morning, but I think you owe me a beer.

 

[Roff]

Hi Ron,
I have run the tests in the last few minutes, couldnt go to bed not knowing.!!

With a 2Vref and a floating mV input it clamps at about 2.5V

I'll re-run the test in the morning, but I think you owe me a beer.

I'm lost. What is our bone of contention here?