Hi ,

I designed a Galvanic skin response meter , it works well as per the circuit attached , The difference amplifier works as per formula . But the voltage divider combination when connected with Input of a Differential amplifier gives values not as per the voltage divider formula , i need to calculate how this voltage generated across the circuit on what calculation basis . Kindly help me . i tried all possible combinations . Hope i miss something basic . Help me please ,. Circuit multisim simulation file screen shot attached .

-- Pragadhees

Biomedical Engineer

INDIA

9585122333

Hello Pragadhees,

The R5-R6 voltage divider is loaded by the inverting amplifier consisting of R1, R2, R7, R8, and U1. Although U1's inputs use only 1pA bias current, a much larger current flows through R7 and R8 to U1's output. We can calculate the loaded divider voltage using Thevenin's Theorem:

Vth) of:Vth = 5v * R6/(R5+R6)= 4.85v (the same as the R3-R4 divider).Rth) is the parallel combination of R5 and R6,which is

Rth = R5*R6 / (R5+R6)= 291.26K ohmV3, the loaded divider voltage you're looking for:V3 = 2.5v + (Vth-2.5v) * R8 / (Rth + R8)= 2.5 + (4.85-2.5) * 330K / (291.26K+330K) =3.748vMultisim analyzes circuits by inverting a matrix of simultaneous equations. Thevenin's Theorem helps by simplifying the circuit so that in this case, no simultaneous equations needed solving -- very handy for manual analysis.

Best regards,

Bob