Hi
i'm trying to do research in the bioimpedance field.
i've built this design of VCCS based of modified howland circuit.
i'm using a signal generator - 1VPP, 1KHZ sinusoidal signal.
By using oscilloscope at the tissue electrodes i'm getting an augmented sinusoidal signal (VPP 1.9v).
But when i'm connecting a multimeter in order to check the microamperes at these electrodes i'm getting a zero result.
it doesn't change when changing the frequency or vpp of the signal generator.
how can it be? there is a sinusoidal voltage but no current.... I know I should get around 39 microampere by this test.

I would be grateful for your help.
Thanks

 

Welcome to AAC...

How are you connecting the multimeter? Show on the schematic...

 

What kind of multimeter?
Most won't measure such a low AC current.
What's the sensitivity and series resistance of the multimeter in the AC current mode for that sensitivity setting?

 

What kind of multimeter?
Most won't measure such a low AC current.
What's the sensitivity and series resistance of the multimeter in the AC current mode for that sensitivity setting?

hi, thanks for the answer
i'm using
AstroAI digital multimeter TRMS 6000
it is capabe of measuring microamperes

 

TRMS6000 can't measure AC current above 400Hz, RTFM

How are you connecting it?

 

As Irving noted, the AC current response drops off above 400Hz (from manual below).

Which terminals are you using for the current measurement and are they connected in series with the signal generator?

 

As Irving noted, the AC current response drops off above 400Hz (from manual below).

Which terminals are you using for the current measurement and are they connected in series with the signal generator?

View attachment 252346

 

hi
i'm using the tissue plus and minus electrodes (at the buttom right) to measure current.

 

Is there any not expensive way to measure microamperes up to 500khz or even 1mhz?

 

i'm using the tissue plus and minus electrodes (at the buttom right) to measure current.

But how are you connecting the meter the electrodes
The meter must be in series with one of the electrodes to measure its current.

 

the multimeter two probes attached to the two tissue electrodes. the electrodes are not connected to tissue at the measurement time.

 

Another question...
When i measure voltage with oscilloscope connected to the tissue electrodes (not connected to tissue) i get a signal with 180 degree phase shift. why is that?
Thanks

 

The gain of your circuit is 1 so with a 1v p_p input signal the current generated across the meter is 1/51k or 20uA p-p or 7uA rms. At 1kHz that probably won't register on the meter. Try it on a DC signal, you might just see something.

 

The gain of your circuit is 1 so with a 1v p_p input signal the current generated across the meter is 1/51k or 20uA p-p or 7uA rms. At 1kHz that probably won't register on the meter. Try it on a DC signal, you might just see something.

Thanks a lot.
Do you have an explanation for the 180 degree phase shift?

 

Thanks a lot.
Do you have an explanation for the 180 degree phase shift?

If the gain is 1, ho do i get a 1.9 Vpp while using 1Vpp signal?

 

If the gain is 1, ho do i get a 1.9 Vpp while using 1Vpp signal?

Good question. What do you think the gain should be and why?

Thanks a lot.
Do you have an explanation for the 180 degree phase shift?

Relative to what? I think I know but can you work it out?

 

Good question. What do you think the gain should be and why?


Relative to what? I think I know but can you work it out?

The gain of the INA128P is 1+50kohm/Rg which is about 2. The gain for the howlland circuit is 1. that's why it is close to 2 (1.9), why isn't it 2 though, because of wires resistance maybe?
I don't understand the 180 phase shift compared to the original signal.

 

I don't understand the 180 phase shift compared to the original signal.

Look at the input polarity designation on your schematic versus that from the 1NA128 datasheet below.

 

Ah, I was referring to the input to R1, not the INA218. The gain from there is 1 assuming R1 = R3 and R2 = R4. The gain of the INA128 is 1+50/51 = 1.980 +/- 0.024% but what's the tolerance on your 51k resistors and your 10k resistors? 1%? so the total tolerance of the circuit end-to-end gain is around +/-2% or 1.98 +/- 0.04, ie 1.94 - 2.02

180deg phase shift: look very closely at the way you are using the INA128...

edit: corrected tolerance calcs.

 

Ah, I was referring to the input to R1, not the INA218. The gain from there is 1 assuming R1 = R3 and R2 = R4. The gain of the INA128 is 1+50/51 = 1.980 +/- 0.024% but what's the tolerance on your 51k resistors and your 10k resistors? 1%? so the total tolerance of the circuit end-to-end gain is around +/-2% or 1.98 +/- 0.04, ie 1.94 - 2.02

180deg phase shift: look very closely at the way you are using the INA128...

edit: corrected tolerance calcs.

You are amazing! Thanks a lot!