Switchable gain op-amp configuration

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
105
The circuit attached is from this App Note.

In my case I need just two gains: unity and 60, so I need only a switch.

In the app note they say:

Gain adjustment resistors are dimensioned such that each range has an amplification of about eight times the previous.
The number of switches may well be increased and the gain difference decreased, however, it is not recommended to
have a larger gain difference than eight between two subsequent ranges. This because gain differences of around ten,
and higher, cause the signal to degrade below 1% accuracy before it can be further amplified.
So in my case with two gains (1 & 60) the difference is more than the recommended. But what's the reason behind this? How is the accuracy affected?

My second question is about coupling capacitor C1. How do I calculate the level of attenuation caused by the high pass filter formed by C1 and R11, R12, R13?
 

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Thread Starter

Blue_Electronx

Joined Jun 10, 2019
105
What is the frequency and amplitude of your signal, and what type of "switch" are you planning on using?
The switch I'm planning to use is like the 74LVC2G66.
The frequency is 50/60Hz and the signal is going to vary from 10 mA to 10 A rms in the primary of the CT. It has 1000 turns, so 10 uA to 10 mA in the secondary. The burden is 30 ohms in my app.
 
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