Op-amp self-enable circuit - Page 1
Forum section: electronics / projects

author: ezalys
author_rank: Frequent Contributor
timestamp: September 10, 2020, 11:23:34 pm
content: I have a circuit with an opamp final output stage, and I'm very concerned with what the opamp does when the circuit powers on, as it will be connected to an extremely sensitive circuit (physics experiment.) I think I'll therefore add an enable line that the microcontroller can twiddle once the circuit has booted up. Does anyone have a suggestion for a good circuit which will retain the opamp's low output impedance but hold the output at zero until an enable line is held high? The circuit is bipolar, for what it's worth.

author: ezalys
author_rank: Frequent Contributor
timestamp: September 11, 2020, 02:03:05 am
content: Analog switch IC seems fine but even they have minimum supply voltages below which I don't know the behavior. If the output of this thing budges 10 millivolts when I apply power, that's bad.

author: Whales
author_rank: Super Contributor
timestamp: September 11, 2020, 02:15:11 am
content: Can you put a high-value resistor between the opamp's output and your delicate device, then short this resistor out after a few seconds using a relay? This will let you avoid shorting the opamp output.

author: ezalys
author_rank: Frequent Contributor
timestamp: September 11, 2020, 03:48:14 am
content: what about an SPDT relay that shorts the output to ground or the opamp output?

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 11, 2020, 05:12:16 am
content: There's also the option of a JFET as a normally closed switch.

author: David Hess
author_rank: Super Contributor
timestamp: September 12, 2020, 12:54:45 pm
content: Disconnecting the operational amplifier output is not a complete solution because starting from an open loop condition will mean the output starts out saturated at one extreme, so doing this can cause the output glitch you want to prevent.Some operational amplifiers have a shutdown control which leaves the output in a high impedance state.Clamping the input signal before applying the supply voltages may be sufficient. This is commonly done in power supply circuits.

author: ezalys
author_rank: Frequent Contributor
timestamp: September 16, 2020, 03:04:36 pm
content: Why would the output start at one extreme? If it's just a buffer output. I was thinking of a circuit like this. It's just driving a sensitive capacitive load that I want to prevent from charging.https://imgur.com/a/hjvoNAU

author: Whales
author_rank: Super Contributor
timestamp: September 17, 2020, 09:35:37 am
content: Quote from: ezalys on September 16, 2020, 03:04:36 pmOpamps are not necessarily stable whilst Vsupply rises from 0V to the desired voltage, and for a short while afterwards. During this time all sorts of assumptions are invalidated, including the common input voltage range and the minimum supply voltage. Internal capacitances are also building up.QuoteWoah, careful! Opamps in negative feedback circuits become unstable when directly supplying capacitive loads. 10mV is nothing, we're talking full oscillation. I recommend testing using a load that approximates your actual device first.May I ask what your load is? Is it a piezo crystal/transducer?

author: ezalys
author_rank: Frequent Contributor
timestamp: September 17, 2020, 12:59:32 pm
content: Van der waals heterostructures. Magic angle twisted graphene in a tunnel structure and another sample has tungsten ditelluride in a tunnel structure.These samples are sensitive and priceless.

author: ezalys
author_rank: Frequent Contributor
timestamp: September 17, 2020, 06:25:48 pm
content: Also, sure, the output starts at one extreme or does other wild things during startup, but you leave the relay open until the output stabilizes at zero. That was the point of adding the relay.