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analogdevicesinc / m1k-cad

ecad for the ADALM1000
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increase A4/B4 feedback resistors to decrease load on 2v5 reference #2

Closed itdaniher closed 9 years ago

itdaniher commented 9 years ago

Common mode error is higher than it should be, due in part to the 750o feedback resistors, forcing the reference, even with bias, to drift from 2v5 due to the 1500o||1500o resistor to 5v with maximum output.

itdaniher commented 9 years ago

There is a reference pin&part compatible with the ADR831. If reference U4 is replaced and biasing resistor R183 is depopulated, this issue abates substantially. Changes implemented by both @mbanciso and myself - 2 units locally here in BOS.

itdaniher commented 9 years ago

"I've done some more thinking about this problem and I think it comes down to the amount of load current variation that the 2.5 V regulator can handle is becoming a limit.

400 ohms assumes 20% tolerance on the 750 ohm resistors 450 ohms might be a better compromise. I measured the R183 on my m1ks and it was around 550 ohms so on the high side near 10% and R24,25 were 720 ohms or so which is on the low side. Happens to be a bad combination.

The 2.5 V ADR381 can source from 0 to 5mA. The change in load current due to the 750 ohm resistors around amplifiers A4 and B4 can be as much as +/- 3.5V/750 or +/- 4.67 mA. ( output of amplifiers can swing from -1 V to +6 V when the loop is open ) With resistor tolerance this is right at the current spec of the part. Now add the pull down resistor to support current sinking 5mA and the part may need to source nearly 10 mA. Twice the spec.

As your data shows we can't increase the feedback resistors without causing stability issues. So....."

itdaniher commented 9 years ago

Implemented upstream. 750 -> 10k.