Instrumental Amplifier (AD620) circuit is not giving correct output

Thread Starter

Samia Riaz

Joined Apr 5, 2022
11
I am working with ad620 (instrumental amplifier) IC. I am using 5 Volts on pin7 and pin4 is grounded to power the IC. I am using 0.38 Volts dc supply on pin3 and pin2 is grounded. The reference pin is also grounded. The gain between pin1 and pin8 is set as 470 ohm. There is an error in output voltage. I am changing the gain but still the output is 0.45 Volts. Secondly, changing the input voltage on pin3 do not change the output. It remains same as 0.45 Volts. If gain resister is removed the output voltage is 0.38v. Where is the problem and what is the solution for it ? Proteus schematics is properly working but problem is in hardware circuit design. Please check the Circuit diagram below.
 

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ericgibbs

Joined Jan 29, 2010
15,542
hi SR,
With a single supply voltage, you need to add a Vref voltage in order to lift the Vout above the 450mV level.

The 450mV Vout is due to using only a single supply voltage

E
 

Audioguru again

Joined Oct 21, 2019
4,696
Your input voltages are far below the minimum allowed voltage range that is shown in the datasheet for the AD620 of +1.9V to +3.8V when the supply is a single +5V.
The values of R2 and R3 are probably too low for your input signal to drive and they are missing an input coupling capacitor which is causing the 1.5VDC calculated to be only 0.38VDC.

Your signal source is not a differential signal so why don't you use an ordinary rail-to-rail opamp?
 

crutschow

Joined Mar 14, 2008
29,825
The minimum requirement, if you are going to use an IC, is to learn how to read its data sheet and understand the limits of the operating conditions.
Working outside those limits, as you have determined, does not usually give the desired results. :rolleyes:
 
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Thread Starter

Samia Riaz

Joined Apr 5, 2022
11
We have solved the problem described above and the new AD620 circuit is attached below, reference voltage is 2.5, Vs+ is 5 Volts at pin7 and Vs- is grounded (pin4) . So we are dividing 5 volts from 2.5 to -2.5 Volts. It is amplifying the signal but the output voltage with respect to ground remains same as 4.2 Volts which should be 5 Volts. Gain is working fine. How can we increase the output voltage?
 

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ericgibbs

Joined Jan 29, 2010
15,542
hi SR,
This is a LTspice simulation of your circuit, what exactly will the Vinput and Vout ranges be, so that I can check the simulation.??
E
EG 1556.gif
 

ericgibbs

Joined Jan 29, 2010
15,542
hi SR,
Checking your circuit with a Ramp input signal, you cannot get a Vout swing greater than Vcc -1.2V, check this clip from the datasheet.

The circuit is no good.
E

Added:
Sim using +/- 5mV test signal, the limits of Vout are plotted.
EG 1558.gif
EG 1557.gif
 
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eetech00

Joined Jun 8, 2013
3,162
hi SR,
Checking your circuit with a Ramp input signal, you cannot get a Vout swing greater than Vcc -1.2V, check this clip from the datasheet.

The circuit is no good.
E

Added:
Sim using +/- 5mV test signal, the limits of Vout are plotted.
View attachment 264807
View attachment 264806
HI E
Your simulation looks a little strange.

What does this mean? :

"Sim using +/- 5mV test signal, the limits of Vout are plotted. "
 

ericgibbs

Joined Jan 29, 2010
15,542
Hi ee,
It is a 5mV thru +5mV Ramp PWL.
The Vout shows the AD620 output, the Gain is set to >>1000.!
So the Vout limits around +/- 1Volt of the 0v and supply voltage, as would expect.
What do you mean by strange.? :)
E
 

eetech00

Joined Jun 8, 2013
3,162
Hi ee,
It is a 5mV thru +5mV Ramp PWL.
The Vout shows the AD620 output, the Gain is set to >>1000.!
So the Vout limits around +/- 1Volt of the 0v and supply voltage, as would expect.
What do you mean by strange.? :)
E
Yes...the gain is 1.498*10^3 !!:eek:

But the output should begin to ramp at the Ref voltage = 2.5.....
 

ericgibbs

Joined Jan 29, 2010
15,542
hi ee,
Check the settings of the TS's supply and Vref settings, they have been tweaked to give a steady state Vout of approx 3.8v and it is Vmax limit. my post #7.

I have set the Vcc and Vref to 2.5V, so that Vout sits at approx +2.5Vout my post #8.

Note the Table for the AD620 on that post.

E
 

eetech00

Joined Jun 8, 2013
3,162
hi ee,
Check the settings of the TS's supply and Vref settings, they have been tweaked to give a steady state Vout of approx 3.8v and it is Vmax limit. my post #7.
Yes. I see that.

I have set the Vcc and Vref to 2.5V, so that Vout sits at approx +2.5Vout my post #8.
doesn't look that way to me.
If VCC =5 and REF = +2.5v, then with the ginormous gain, the output should ramp up at 2.5 and flatline at vcc-1.2

I'm thinking there is something wrong with the model....
 

ericgibbs

Joined Jan 29, 2010
15,542
If VCC and REF are both +2.5v, then with the ginormous gain, the output should ramp up and flatline at vcc-1.2

hi ee,
It does.

The Ramp is Vcc-5mV up to Vcc +5mV in the post #8 circuit.

Draw the circuit post #8 in LTspice and run it.

I am on another PC which does not have that asc file, I will post my copy tomorrow morning.
E
 

eetech00

Joined Jun 8, 2013
3,162
If VCC and REF are both +2.5v, then with the ginormous gain, the output should ramp up and flatline at vcc-1.2
I corrected this statement. It should have read Vcc=5 and REF=2.5

It does.

The Ramp is Vcc-5mV up to Vcc +5mV in the post #8 circuit.
Are we referring to the same post? post#8?
I see a graph with an output ramping from ~1v to ~3.9v

With VCC=5 and REF=2.5, the output should start at 2.5v and flatline at 5v-1.2v

Draw the circuit post #8 in LTspice and run it.
I am on another PC which does not have that asc file, I will post my copy tomorrow morning.
E
I'll wait for your post.
 

ericgibbs

Joined Jan 29, 2010
15,542
Hi ee,
I don’t see why you are not following the circuit simulation.?

Case1:
The V3 input signal is set for 2.5v, into +in of the AD620
the +2.5V ref input is at +2.5v, pin –in of the AD620
That means the CMV is +2.5v

So the Differential Voltage input to the AD620 is 0v.

If the Vref pin of the AD620 is connected to 0V, the Vout of the AD620 would be just above 0V.
But the TS has set the Vref Pin to +2.5v, this lifts the Vout to +2.5v
The steady state of the circuit.


Case 2:
If I now set V3 to 2.501V , an Increase of +1mV
[ In order to keep the circuit sensible, I have set the gain to 500]

The Differential input is 1mV, so with a gain of 500, the Vout will be +2.5 (+0.5v) = +3v.


Case 3:
If I now set V3 to 2.499V , an decrease of -1mV
[ In order to keep the circuit sensible, I have set the gain to 500]

The Differential input is -1mV, so with a gain of 500, the Vout will be +2.5 -0.5v) = +2.5v.

If the mVolt Inputs change over a wider range the Vout will run into the limits of the AD620 output swing range.

Ignore the 'apparent' noise on the Vout, suspect it is a LTS artefact.

E

EG 1563.gif
 

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ericgibbs

Joined Jan 29, 2010
15,542
Because there is a lot implied by post #8 that isn't there...
Hi ee.
As you are somewhat of an expert with LTS, could you tell me exactly what was 'implied' in post #8.??

It has all the labels and voltages in post #8, that are used in the three Cases I posted, the only exception is the over the top Gain figure, reduced to 500.

All that the 'Cases' that I have posted, just show in more detail how post #8 circuit functions.

E

Added: Post 8 with same set up as the Cases, same result as #8

Footnote:
You also posted that I stated as a quote , which not so.!
If VCC =5 and REF = +2.5v, then with the ginormous gain, the output should ramp up at 2.5 and flatline at vcc-1.2
This is your statement from post #13.


EG 1564.gif
 
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eetech00

Joined Jun 8, 2013
3,162
Hi ee.
As you are somewhat of an expert with LTS, could you tell me exactly what was 'implied' in post #8.??
The three "cases" you've posted in #16.
I wasn't able to glean all of that from post #8. But you've explained it all and I thank you for that.

Footnote:
You also posted that I stated as a quote , which not so.!
If VCC =5 and REF = +2.5v, then with the ginormous gain, the output should ramp up at 2.5 and flatline at vcc-1.2
This is your statement from post #13.
I did not state that you stated that. I made the statement and it is not quoted. And I even made a correction to it in a later post (#15).

At any rate....

VCC in all of your posts is not 2.5v, it is 5.0v (two 2.5V voltage sources in series =5.0v).

Thanks again for your explanation in post #16.
This is my last post in this thread.
 

ericgibbs

Joined Jan 29, 2010
15,542
VCC in all of your posts is not 2.5v, it is 5.0v (two 2.5V voltage sources in series =5.0v).
Hi ee,
Nowhere do I state that the supply to the AD620 = 2.5v, it is obviously Vref =2.5v and Vcc = 2.5v in series, which make the AD620 supply = 5v, check the images I have consistently posted on this Thread.

It appears you do not understand the circuit, especially the operation of an INA, AD620 amplifier.
That's not a problem, but don't try to pass the buck back to me for your lack of understanding.

E
EG 1566.gif
 
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