Designing LM2902 as difference amp

Knighty Nimp

Other Parts Discussed in Thread: LM2902

Hello,

Can anybody give the equation to calculate acculate VO which included all factor eg VIO, etc(refer schematic as attached)

Note: we used Vo=(V2-V1)Rf/Ri equation but seem not acculate enough.7230.LM2902.pptx

over 12 years ago

0 Ron Michallick over 12 years ago

TI__Guru**** 166106 points

Knighty,

Here is a good reference book. See page 98 for diff amps formulas
http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slod006b

Errors in resistor ratios will reduce the common mode rejection.
The DC offset set error from VIO is VIO*(1+Rf/Ri).

0 Knighty Nimp over 12 years ago in reply to Ron Michallick

Expert 1260 points

Ron,

Thanks,

Opamp data sheets specify the CMRR for the opamp itself, but the opamp differential amplifier circuit also has a CMRR, so how can we considering CMRR into my circuit design base on LM2902 device?

0 Jason Seitz over 12 years ago in reply to Knighty Nimp

TI__Intellectual 2630 points

Knighty,

The following equation can be used to estimate the CMR performance (using your labels) assuming matched resistors:

CMR = 20log [(1 + Rf/R1)/Tol]

Where Tol is the resistor tolerance in fractional form. For example, a difference amplifier with a gain of 10 V/V where Rf = R3= 100kΩ, R1 = R2 = 10kΩ and Tol = 1% = 0.01 will have a CMR:

CMR = 20log [(1 + 100kΩ/10kΩ)/0.01] = 61dB

Amplifiers

Amplifiers forum

Designing LM2902 as difference amp