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I have the following schematics:

enter image description here

If I have a scope probe on VIN, everything works very well. VIN is an amplification of the input and the ADC reports it very well: good accuracy and no delay. But if I don't put the scope probe on VIN, the measurement is problematic and if input drops VIN takes a lot of time to follow. In other words, the scope makes work the circuit. Any idea what I should modify?

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1. Naming an output node as "VIN" is somewhat confusing. 2. A proper schematic that shows amplifiers as amplifiers would make it much easier to see your problem. – The Photon Nov 25 '12 at 18:14
If you don't have a symbol for TLV2782 in Altium just use any other double opamp IC like TL072 and change the name and footprint. – Szymon Bęczkowski Nov 25 '12 at 18:56
Input is an ultrasonic sensor. D5 and D6 was the idea to do a rectifier to have a clean signal to measure. – gregoiregentil Nov 25 '12 at 20:56
In that case get rid of D5, it's doing nothing for you. – The Photon Nov 25 '12 at 21:49
I have removed D5 but still the same: works on the scope, but without it, the ADC report shows a very slow fall. I have also tried to replace D5 by a 2M resistor, but still the same. Any advice? – gregoiregentil Nov 25 '12 at 22:08

Diode D6 means that the op-amp can only drive the ADC input high. On its own, the amplifier circuit can't pull VIN low.

When you connect the scope probe, you effectively add a resistor to ground, probably in the neighborhood of 1 - 10 Megohms. This pulls VIN low whenever the op-amp isn't driving it high.

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Additionally D5 will never conduct as opamp's output voltage will always be equal or above GND. – Szymon Bęczkowski Nov 25 '12 at 18:59
@SzymonBęczkowski, D5 could be there as a protection against turn-on transients or something. But without more context its hard to know exactly what the designer had in mind. – The Photon Nov 25 '12 at 19:19
Input is an ultrasonic sensor. D5 and D6 was the idea to do a rectifier to have a clean signal to measure. – gregoiregentil Nov 25 '12 at 20:57
Your opamp will not output negative voltage so you don't need these two diodes. Diodes will not clean your signal. If you are having problems with noise in your measurement signal just slow down your amplifier. – Szymon Bęczkowski Nov 26 '12 at 8:16

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