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...)
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
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...)
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.
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.
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.
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
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?
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.?
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.
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.
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.
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!
. 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...