# AD620 output diamond plot and some questions

#### po210

Joined Dec 24, 2013
23
Hello

I have the circuit shown i this white board picture
As you see, my single rail power supply is 9V and i get a 1V output for a sinusoidal 10mV input, at gain of 100,as expected.( the gain resistor is not 1k as is shown in drawing, but 530 ohm)
Notice the input signal has no Vcm (common mode) component. I just connect directly my blue 10mV sinusoidal input (from signal generator,no DC level) through two 470ohm resistors.
If i check the expected output for this scheme with AD diamond plot tool i get this:

So you see a perfectly centered signal inside operational limits.
But to do so, notice i had to set a common mode Vcm=4.5V...which is not what i had in first picture in my real board. In fact, as i understand I have Vcm=0V there!!
So, why is my circuit "working"?. If i put Vcm=0 in the diamond plot tool, i get error messages, with part of signal excursion out of good operational limits.
Also, why i get an output signal centered on 2.75V instead of 4.45 which is the VRef I used? How could i measure my real Vcm? As far as i know, ad620 needs 1.6V of common mode to work properly. 4.45 - 2.75 = 1.7V...ummm...close enought, but i don't know if this is just a coincidence or how to play with this.
Thank you.

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#### ericgibbs

Joined Jan 29, 2010
15,673
hi,
The AD620 is designed for dual supply operation, it will work with a single supply but there are limitations

The problem when using a single supply with a AD620 that the Vout will never be able to be less than approx +0.7V.

E

#### po210

Joined Dec 24, 2013
23
What i see is that output signal appears centered in a DC level (relative to ground) that depends on gain, and not exactly on Vref. The values for signal input resistors and EMI capacitors have no influence setting that DC level.
Also, i can't measure Vcm by attaching osciloscope probes to the imput/ground and substracting channels without seriously destroying the input signal.

#### ericgibbs

Joined Jan 29, 2010
15,673
hi,
Is the 10mV signal from the SG , AC coupled.?
If yes, there is no DC path for the OPA input currents.!

E
Try this option.

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#### po210

Joined Dec 24, 2013
23
Thanks for the effort Eric.

Coupling was DC,but i tried your circuit anyway, but at Vcc=4.5V I get a totally distorted signal. At supply 9V ,with Vref=2.25 I see just a constant DC 2.25V output...but as soon as I remove the 1Kohm resistor to ground I see the sinusoidal signal again,but centered in 1.5V. (not 2.25)
Also, i see the caps are essential in order to have a decent signal without a lot of noise.
I see you used DC coupling. Then, what is 1K resistor role?

#### ericgibbs

Joined Jan 29, 2010
15,673
hi po,
Do you mean the 10k , R4 from the -In, that is intended to provide a DC ground path for the AI inputs.
Please post you latest complete circuit and we can check it out.
Show any external input devices.
E

#### po210

Joined Dec 24, 2013
23
Yes sorry,I meant that 10K resistor. I thought it wasn't needed because the signal was directly attached to ad620 inputs, but maybe that does not imply a path to ground,isn't it?
Here is the circuit, which is essentially the same, with actual values.
About the scope, it shares ground with the power supply.As i told, each time i put red 10K resistor, output goes to Vref but there is no signal on ir.

#### ericgibbs

Joined Jan 29, 2010
15,673
hi po,
How is the SigGen, 10mVac source connected to the two 470R input resistors?
E

#### po210

Joined Dec 24, 2013
23
hi po,
How is the SigGen, 10mVac source connected to the two 470R input resistors?
E
Yes.

#### ericgibbs

Joined Jan 29, 2010
15,673
I know its connected, but how is it connected.??

#### po210

Joined Dec 24, 2013
23
The SG is a cheap Feeltech ,with two cables,one red the other black. Those cables go directly to the inputs.

#### ericgibbs

Joined Jan 29, 2010
15,673
hi po,
So that suggests that one of the SG inputs is at Gnd/Common and so it grounds one input of the AD620.?

If the SG has a 50R output impedance, I would suggest you use a coupling capacitor on the other input lead to the AD620, to block any DC offset that maybe present on that SG output.

Also have the 10k from the AD620 pin to 0v that has the DC blocking cap.
E
Try this setup.

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#### po210

Joined Dec 24, 2013
23
Thanks Eric.Sorry for the delay.
I tried your last suggestion but I only get a constant perfect Vref DC as output. Tested also in different breadboards with same results: as soon as a resistor in an input goes to ground, output goes to Vref.

Anyway, I'd like to use this as ECG monitor, so I need "floating" inputs not referenced to ground to test this .How could i simulate that if as it's seems one of my signal generator will always put a ground on an AD620 input? (Which maybe has the effect of biasing output down,so is not centered in Vref?)

#### po210

Joined Dec 24, 2013
23
I also tried putting two DC blocking capacitors at inputs, with a return to GND path resistor,and ir happens the same. No signal output

#### ericgibbs

Joined Jan 29, 2010
15,673
hi po,
That should give an output signal.
I don't have a AD620 in stock to a bench test.
Do you have second power supply that you could use with the AD620 as a dual power supply.
As you know the AD620 is designed for dual supplies, I know it will work with a single supply but certain input conditions are required.
Have you checked an ECG circuit.?
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