Need help understanding a schematic

[Prithul0218]

I was looking at this schematic of a DC Load that Scullcom Hobby Electronics made. I couldn't figure out why they were using U7C. The OPA277 has fairly low output impedance, enough for an ADC, doesn't it?

I am asking it here cause Scullcom Hobby Electronics didnt reply.
Thanks.

Schematic source: http://www.scullcom.uk/design-build-an-electronic-dc-load-part-1/

[anishkgt]

He may have explained it in his video.


Www.Georgehobby.wordpress.com

Equipments: DSO104z, Hakko FX888D, Brymen BM869s

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[Prithul0218]

I went through all the videos. I don't remember that.

[Zero999]

I was looking at this schematic of a DC Load that Scullcom Hobby Electronics made. I couldn't figure out why they were using U7C. The OPA277 has fairly low output impedance, enough for an ADC, doesn't it?

I am asking it here cause Scullcom Hobby Electronics didnt reply.
Thanks.

Schematic source: http://www.scullcom.uk/design-build-an-electronic-dc-load-part-1/

It's just a way to use a spare op-amp in a quad package. It could be removed but then the spare op-amp would need to be configured as a unity gain buffer, with its input tied to 0V. If you're using separate op-amps, rather than the quad AD9630, then remove U7C and R11.

[mvs]

The OPA277 has fairly low output impedance, enough for an ADC, doesn't it?

More then enough to burn something downstream... I would put a resistor to limit current if the output goes negative (Vee is -5V). MCP3426 has diode clamps with 2mA abs. max rating, for AD8630 i have not found any data.

[MasterT]

OPA277 is not R2R, so he used dual power - and had a problem to interface with single power ADC.  So he put single powered buffer, though that not solved an issue with input injected current from negative rail (as pointed already).

[Prithul0218]

So I guess I was right, it has no use.

Thanks.

[MasterT]

If you remove U7C, recalculate R5: -5V /2mA = 2.5 kOHm minimum value. Do your homework, calculate an error that R5 would introduced. See if it's acceptable for 13 bits (1/2  bit).
If it's to me, I'd ditch both OPA277 and U7C, and put MCP6V27 as an ADC buffer, differential to single ended converter.
 

[Prithul0218]

If you remove U7C, recalculate R5: -5V /2mA = 2.5 kOHm minimum value. Do your homework, calculate an error that R5 would introduced. See if it's acceptable for 13 bits (1/2  bit).
If it's to me, I'd ditch both OPA277 and U7C, and put MCP6V27 as an ADC buffer, differential to single ended converter.
 

I see. I guess cost comes to play.

[MasterT]

Look at the circuits for DC error sources:
1. input resistive divider (50-100 ppm/T),
2. buffer amp (AZ),
3. second amp interfacing.
The author solved a "zero set" error at 2, but introduced it back at 3, since minimum output voltage for single power is newer zero.
There are also many different problem, like full scale error, drift in the ADC itself and resistors, that AZ opa egotistically doesn't care. So what is the point to spend money on AZ, if overall accuracy is no difference to low cost INA?
I would put a reed / opto relay right at the front to re-calibrate periodically , though price for relay could make this approach less cost effective.

[netdudeuk]

I am asking it here cause Scullcom Hobby Electronics didnt reply.

I like his channel but there have been no new videos for a while. Hope he’s ok.

[Prithul0218]

I am asking it here cause Scullcom Hobby Electronics didnt reply.

I like his channel but there have been no new videos for a while. Hope he’s ok.

Yeah, he's not replying to comments either.