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

View attachment 252415

Thanks a lot!

 

Hello friends!
After i've tested this circuit attaching the "tissue electrodes" to a single resistor (i've tried 1.5Kohm, 21Kohm and 51Kohm, each one by itself) - it seems that i'm getting constant current only if the test resistor is 1.5kohm. with the 21Kohm and 51Kohm resistor i'm getting lower and lower current as I increase the frequency from 500Hz to 500Khz gradually.. It doesn't occure with the 1.5Kohm resistor.
I can't relly on this circuit because i want to use it for bioimpedance measurment and i need a constant current pump for different resistances and frequencies.
Any Ideas why is that occuring and how can I solve it?

 

Your problem is that you have frequency dependent component in the gain feedback path of which reduces the gain as the frequency increases.

This is what happens when you use a circuit without fully understanding how it works.

 

I know. I'm a medical doctor not an engineer. I'm doing my best to learn electronics as i want to use bioimpedance research. It's not simple...

 

Bioimpedance isn't simple.... where are you doing your research?

C2 reduces the gain at higher frequencies leading to the rolloff... see below (green=1k5, blue=20k, red=51k load) with and without C2. The LF412 opamp rolls off above 100kHz as well.

 

thanks a lot for your answer.
It doesn't work for me though...

 

Hello
I need help designing a VCCS.
The input will come from a function generator - 1Vpp at frequencies 1Khz to 1Mhz.
This voltage would go through a buffer and then enter the VCCS.
The vccs should give a constant current of <1mA, it should be constant in a wide range of frequencies 1Khz-1Mhz, and when measured on a range of loads (10ohms - 10Kohms).
I'm attaching a diagram of VCCS based on modified howland circuit, but it does not give a stable current at those frequencies or loads.
I'd be glad if you could guide me about the chips and resistors/capacitors needed to get this kind of VCCS.
Thanks a lot.

 

hi yoni.
This is your circuit in LTspice.
What are the typical signal levels from the signal generator.

Also, what is the Electrode load impedance.?
E

 

Hi Eric, where did you get the INA128 model?

Here's the TS's LF412C model, use with opamp2 symbol.

 

hi @Irving
Files for INA128 zipped
E
Let me know if you have a problem

 

The circuit works fine 1k - 1M, 10ohm - 100K

You need better amps, eg
change INA128 to AD8421
change LF412 to AD8066
change all 10k to 100k
delete C1
change gain set resistor to 10k for g=2

The circuit contains high impedances so layout to get bandwidth and stability will be important. a double sided PCB with a ground plane is likely to be essential.