Hi,
Hope everyone is doing great.

I am building a circuit in LTSpice using the inverting configuration of the LT1028 operational amplifier. The input signal provided is 0.7 Vp, which is applied to R1 (5M ohm). For the feedback resistor (Rf), I am using my biosensor, whose impedance varies from 10M ohm to 25M ohm. The equivalent circuit of my biosensor is attached in the picture. I want to measure the value of Rf using the formula:

Rf= R1 x ( −Vout/Vin)

After R1, the voltage drops from 0.7 Vp to 60 uV before reaching Rf (the biosensor).

I have created a separate circuit in LTSpice representing the equivalent circuit of my biosensor. The values I used for R and C in this LTSpice circuit are based on the impedance analyzer readings. The actual impedance of my sensor is 15M ohms, but I used the values of R and C provided by the impedance analyzer, where R and C are in parallel. The voltage applied to this circuit is 60 uV, but the output I obtained from the circuit is very low (around 0.8 uV), and the waveform after the circuit is not a sine wave.

Given this, the impedance value for Rf that I calculated is incorrect.

I have a few questions:

1. Did I correctly build the equivalent circuit of my biosensor in LTSpice? If not, could someone suggest the necessary changes to the circuit?
2. Why am I not getting the correct value for Rf when I use the equivalent LTSpice model? When I use a simple resistor for Rf, I can calculate the correct value using the above formula.
3. Can someone explain why the LTSpice model of my equivalent circuit alters Vout and why the parallel capacitor significantly affects Vout?

For reference, I will attach the picture and the LTSpice ASC file.

 

Equivalent:

 

hi zain,?
Have you checked the frequency response of the equiv circuit at your test 1kHz signal?
I make it -30dB down,
E

 

Equivalent:
View attachment 331050

Dear Sir,

Thank you very much for the reply. I will look into the 1022.

First, the equivalent circuit I made yesterday was incorrect. sorry for correct me

When I use this equivalent circuit, which represents my biosensor in my complete circuit having inverting amplifier configuration, I get a very high voltage (-4.9V). Based on that, the impedance of my biosensor, as per the calculations, is 35MΩ, whereas it should be 15MΩas per impedance analyzer results.
I am attaching the result after adding this circuit as Rf in my circuit

 

hi zain,?
Have you checked the frequency response of the equiv circuit at your test 1kHz signal?
I make it -30dB down,
E
View attachment 331066

No i didn't check this response, because my circuit was totally wrong, when i read in detail, then i realized that my equivalent circuit was wrong.

 

No i didn't check this response, because my circuit was totally wrong, when i read in detail, then i realized that my equivalent circuit was wrong.

Dear Sir.
Yes i checked the gain (magnitude of output/input) at 1kHz is around -30 dB. which shows my signal strength become very week/attenuate at 1Khz in this my circuit. I totally agree with you. But after made the correct circuit and build it in the LTSPice it increase the amplitude of signal from 1Vpp (Vin) to -4.9Vpp(Vout) and according to this Vout, biosensor impedance is 35M ohm which is actually 15M ohm in real circuit.
Moreover, the signal that i provide to this Circuit become very strange. I am going to attach the schematic for your consideration.
I dont know how to attach the asc. file so i am going to attach the pictures showing the complete circuit, V at point 1 and vout.
The problem is the voltage is remain sin the negative and not swing, moreover this is very high Vout