Sapphyre Group Return Diff Amp CMRR

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Sapphyre Group Return Diff Amp CMRR

Postby rmorris » Thu Mar 16, 2017 9:25 am

Hi. First of all - Hello - as I've just registered here though have often looked in the past.
So a question:
Soundcraft Sapphyre:
I've been looking at schematics:
http://www.technicalaudio.com/pdf/Soundcraft/Soundcraft_Sapphyre/Sapphyre_Schems_Only.pdf
and while most things make sense to me I'm a bit lost on the Group Insert Returns.
'Differential Amp' stage but with slight ratio 'mismatching' on the resistor values indicated eg 3K9 rather than 3K75 (7K5/2)
By my calculation that limits CMRR to around 38dB ???
Anything to do with equalising current in each leg ?
Would be interested to know the design considerations here...
Thanks
rmorris
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Re: Sapphyre Group Return Diff Amp CMRR

Postby Jim Williams » Thu Mar 16, 2017 11:16 am

Good calculation, it's designed for -40 db CMRR, with 1% resistors. Soundcraft used 2%. Check on-line apps from the electronic trades and apps from the chip makers, they have all sorts of diff amp designs.

Some articles discuss the different impedances read by the inverting vs non-inverting inputs, some resistor adjustments can improve CMRR.

In the biz, the 3 opamp instrumentation amp is the most commonly used topology, those can do around -80 db CMRR if well trimmed. There are also many pre trimmed chip solutions like the balanced input recievers made by TI and AD.

In the studio, it's not all that important when you are interfacing dozens of tie lines and the system noise is already low. Usually ground currents are the largest problem.
Jim Williams
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Re: Sapphyre Group Return Diff Amp CMRR

Postby rmorris » Fri Mar 17, 2017 7:26 am

Thanks for the input Jim.
Yeah - I know what you're saying about various diff amp / instrumentation amp topologies.
I can design these things - although tbh it almost inevitably ends up with an integrated ic solution because it works out more economic than external precision trimmed resistors and op amp to suit as well as pcb area.
Also agree that it's 'ground currents' that are the real issue here esp at line (cf mic) level.
But I was really wondering why it hadn't been designed with the nominally matched resistor ratios as that doesn't seem any more of a problem.
Wondering whether it was allowing for any departure from ideal op amp behaviour with TL07x. I don't think so but there's always more to learn...
rmorris
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Re: Sapphyre Group Return Diff Amp CMRR

Postby Jim Williams » Fri Mar 17, 2017 10:29 am

Try these resistor ratios. Use a 10 k for the inverting input and feedback. Use a 4.75k for the non-inverting input. Use a 5.23k from the non-inverting input to ground. This compensates for the impedance the non-inverting input sees to ground. Alternatively, use a pair of 4.99 k resistors on the non-inverting inputs.

The opamp wants to see the same impedance off each input, This creates the same resistance input/load off each input pin. Inverting sees 10k ohms, non-inverting sees 10 k ohms when the 4.75k and 5.23k are added. Trimming that shunt resistor can maybe improve CMRR a few more db's.
Jim Williams
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Re: Sapphyre Group Return Diff Amp CMRR

Postby rmorris » Tue Mar 21, 2017 8:07 am

Yes - that's more like I was expecting to see.
Although with those particular resistances it's unity gain cf the '-6dB' of the Sapphyre circuit.
tbh I've never found it particularly crucial to equalise the 'leg impedances' in either audio or other applications.
But it's a good while since I was sat in front of an AP System 2 :-)
Thanks for the balanced input (pun intended)
rmorris
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