SaM - CMRR • Why We Need a Good Differential Amplifier

Remeber:

Consider a simple SaM - Operational Amplifier, used as a SaM - Differential Amplifier, however non-ideal, specifiaclly we consider these internal resistances:
Also attached to this amplifier an half bridge:
If we consider $R_{10}=R_{20}=R_3=R_4$, or $K=1$, so that we can maximize the sensitivity, we will have that the output of this simple bridge+amplifier is equal to:$V_0^{\prime }=A_d{V}_d+A_c\left(V_{CC}+\frac{V_d}{2}\right)$If we talk about strain gauges, my “informative part”, $V_d$ is really small ⇒ I’ll have a very small variation of the signal $V_d$ (maybe about some millivolts or even microvolts). This is the reason why I need a good differential amplifier Also a good differential amplifier is an amplifier with high CMRR, ideally infinte.

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Why We Need a Good Differential Amplifier?

We consider a simple scheme of a differential amplifer, considering some non linearities as the common mode resistance ($R_C$) and differential resistance ($R_D$).

• $V_c$ is the voltage on $R_{C1}$ (the common mode resistance)
• $V_d$ is the voltage on $R_d$ (the differential resistance)

We attach a resistive bridge before it:

If we consider $R_1=R_2=R_3=R_4$, so $K=1$, we will have that the output of this simple bridge+amplifier is equal to: $V_0^{\prime }=A_d{V}_d+A_c\left(V_{CC}+\frac{V_d}{2}\right)$ If we talk about strain gauges, also my “informative part”, $V_d$ is really small, so I’ll have a very small variation of the signal $V_d$ (maybe about some millivolts or even microvolts). This is the reason why I need a good differential amplifier. Also a good differential amplifier is an amplifier with high CMRR, ideally infinte.