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| Input signal appearing on the +Vpin of an opamp |
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| Yamin:
--- Quote from: Zero999 on September 17, 2020, 12:25:28 pm --- --- Quote ---So to troubleshoot the unit I am working should I have a look at the decoupling caps on the power rail? Should I add a cap on the power rails to see if I could rectify the problem I am having? If so what would be a good value to check? Thanks so much --- End quote --- Check the connections to the power supply first. What sort of power supply are you using? --- End quote --- Well I am working on a practice guitar amp, there is tons of noise on the final output. Not even possible to hear the input signal. I'm troubleshooting around the pre amp section. There is a DSP section inbetween the final output and the preamp. The power supply is a 19V power brick (I have tried different power supplies). There are some LDO regulators and the 12V is derived from there. --- Quote from: Zero999 on September 17, 2020, 12:25:28 pm ---Yes, it's posible to design an op-amp circuit like that, so the output and power supply rail share the same connection. --- End quote --- Could you please explain to me why that would be the case (sorry to sound like a noob). Thanks again. |
| TimFox:
--- Quote from: Zero999 on September 17, 2020, 12:25:28 pm --- --- Quote from: TimFox on September 16, 2020, 03:23:18 pm ---If one op-amp impresses that much input signal on a power-supply line, then you will see cross-talk (unwanted coupling) to any other amplifiers sharing that power-supply line. --- End quote --- It depends on who sensitive the circuit is. Normally the power supply rejection of the op-amp will handle variations in power supply voltage. --- End quote --- Yes, a good op-amp has a decent PSRR, but now that single-supply op-amps are prevalent, the required bias divider (as in the OP's circuit) can easily couple more voltage to the non-inverting input than the equivalent offset from PSRR. |
| Zero999:
--- Quote from: TimFox on September 17, 2020, 02:54:12 pm --- --- Quote from: Zero999 on September 17, 2020, 12:25:28 pm --- --- Quote from: TimFox on September 16, 2020, 03:23:18 pm ---If one op-amp impresses that much input signal on a power-supply line, then you will see cross-talk (unwanted coupling) to any other amplifiers sharing that power-supply line. --- End quote --- It depends on who sensitive the circuit is. Normally the power supply rejection of the op-amp will handle variations in power supply voltage. --- End quote --- Yes, a good op-amp has a decent PSRR, but now that single-supply op-amps are prevalent, the required bias divider (as in the OP's circuit) can easily couple more voltage to the non-inverting input than the equivalent offset from PSRR. --- End quote --- The original poster hasn't posted a schematic. Are you referring to the one I posted? If so then yes, it doesn't have great power supply rejection, but that's easy to resolve by increasing C1 and/or adding another RC filter before the non-inverting input. --- Quote from: Yamin on September 17, 2020, 02:40:06 pm --- --- Quote from: Zero999 on September 17, 2020, 12:25:28 pm --- --- Quote ---So to troubleshoot the unit I am working should I have a look at the decoupling caps on the power rail? Should I add a cap on the power rails to see if I could rectify the problem I am having? If so what would be a good value to check? Thanks so much --- End quote --- Check the connections to the power supply first. What sort of power supply are you using? --- End quote --- Well I am working on a practice guitar amp, there is tons of noise on the final output. Not even possible to hear the input signal. I'm troubleshooting around the pre amp section. There is a DSP section inbetween the final output and the preamp. The power supply is a 19V power brick (I have tried different power supplies). There are some LDO regulators and the 12V is derived from there. --- End quote --- What sort of noise is it? A hum, buzzing noise or hiss? --- Quote --- --- Quote from: Zero999 on September 17, 2020, 12:25:28 pm ---Yes, it's posible to design an op-amp circuit like that, so the output and power supply rail share the same connection. --- End quote --- Could you please explain to me why that would be the case (sorry to sound like a noob). Thanks again. --- End quote --- The main reason would be to simplify wiring, as only two conductors are needed, or a single wire if you can use earth return, but it makes the op-amp circuit more complex. Here's an example. The output of the op-amp is connected to its positive supply, which is connected to a power supply, via a resistor. The power supply and resistor could be at one end of a piece of cable, with the amplifier and traducer at the other. The amplifier has two different gains: DC and AC or signal frequency. At DC the op-amp has a gain of 1+R2/R1, which amplifies the voltage on D1 by 11, causing the output to sit at just over 6V. At AC, R3 is in parallel with R2, making 100k, so the amplifier has a gain of 2. For higher AC gains than 11, put R3 and C3 across R1. |
| TimFox:
Yes, I was referring to your posted schematic, which is typical for single-supply operation of op-amps. (I was looking for a typical circuit and saw yours, but hastily referred to it as the OP's.) PSRR of single-supply circuits, including bias chains, is an important design consideration. I was involved in a re-design of a system with over a thousand dual-rail op-amps at the end of long power distribution wiring, and we had to add point-of-load regulators to avoid cross-talk from one end of the chain to the other due to power supply impedance. (This was due to the PSRR of the op-amps themselves, and the class-B output stages pulling relatively large currents from the supply at high output voltage, thus affecting the voltage at other amplifiers far away, resulting in non-local crosstalk.) |
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