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Old 5th January 2013, 06:40 PM   #91
RNMarsh is offline RNMarsh  United States
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Happy New Year 2 all !!

And is there a CMR # for the circuit?
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Old 6th January 2013, 06:03 AM   #92
dadod is offline dadod  Croatia
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Quote:
Originally Posted by RNMarsh View Post
Happy New Year 2 all !!

And is there a CMR # for the circuit?
I am not sure how to simulate it with this circuit as here is only one input, maybe you can help me with it?
I have one channel working now, and preparing second one.
Damir
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Old 6th January 2013, 07:40 AM   #93
RNMarsh is offline RNMarsh  United States
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At R9 would be a (-) input port to use for cmr tests.
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Old 6th January 2013, 09:11 AM   #94
dadod is offline dadod  Croatia
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At R9 would be a (-) input port to use for cmr tests.
Thanks, I will do that.
Here are some photos of the GainWire boards.
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File Type: jpg 06012013034.JPG (155.9 KB, 145 views)
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Old 7th January 2013, 08:47 AM   #95
dadod is offline dadod  Croatia
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I am not sure I did it correctly, but here it is, CMRR.
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Old 7th January 2013, 03:37 PM   #96
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Put the second AC source in series with R9. By bypassing R9 your circuit is running at full gain. CMRR will be higher at a normal gain.
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Old 7th January 2013, 04:36 PM   #97
dadod is offline dadod  Croatia
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Originally Posted by keantoken View Post
Put the second AC source in series with R9. By bypassing R9 your circuit is running at full gain. CMRR will be higher at a normal gain.
I've got lower!
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Old 7th January 2013, 04:55 PM   #98
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Hmm. Replace R8/R7 with CCS?
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Old 7th January 2013, 05:15 PM   #99
RNMarsh is offline RNMarsh  United States
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CMR uses same source (1) to drive both inputs -- so use 2 series R from sig gen of maybe 1K or more going to + and - input ports. Looking for a difference plot. Should have >60dB rejection or cancellation between ports for optimum performance. SOTA would be >-100dB.
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Old 7th January 2013, 05:42 PM   #100
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Maybe true for modular instrumentation amplifiers but this circuit will only be connected one way in one circuit. It has passed all the requirements so far so even if CMRR is low, there may be little to gain from increasing it with this particular implementation.

R7/R8 give the + input slightly extra gain. At a current gain of 18.54k at the Jfet sources, this calculates to a CMRR of 2.85db, which seems to agree with the simulation. R7/R8 need to be bootstrapped to the negative input or replaced with current sources.

I think in the first simulation since you bypassed the 1k resistor you caused major offset which saturated/clipped the output resulting in optimistic PSRR.
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