inverting amplifer virtual ground (negative terminal) power on voltage? (digipot - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 26, 2014, 06:35:22 am
content: How does the virtual ground of an op-amp look like as a transient response? Typically we write that the negative terminal of an op-amp in inverting configuration should converge to 0. But is it always zero during startup and power down? I want to use a 5V max digipot in a +- 15V circuit. Initially I thought it would be OK because I knew that it "should" be zero.I don't have the exact op-amp I plan on using in my design to test, idk if this varies based on op-amp design or what. I could do a test here but I don't know if the results would be universal for all.have I designed in failure?I wanted 10k - 10k digipot - 10k where the tap on the digipot connects to the inverting terminal of the op amp.. but I am starting to think something is amiss...do i actually need a +-15V bipolar digipot? must it be bipolar if I am working with amplifying a AC signal? how bi polar must it be?I think I could possibly put Gnd in my 5V digipot as -0.3 volts. maybe.

author: LvW
author_rank: Frequent Contributor
timestamp: April 26, 2014, 08:00:40 am
content: I think, the concept of "virtual ground" applies for the steady-state only - that means: After the feedback loop has settled.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 26, 2014, 08:38:25 am
content: Quote from: LvW on April 26, 2014, 08:00:40 amdoes it settle inthe same way in all amplifiers?

author: LvW
author_rank: Frequent Contributor
timestamp: April 26, 2014, 08:54:17 am
content: Quote from: SArepairman on April 26, 2014, 08:38:25 amOf course, it depends on the opamp delay properties as well as on the kind of the feedback path (with/without reactive elements).

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2014, 06:43:38 pm
content: You can calculate the virtual ground impedance versus frequency from the operational amplifier's open loop gain, gain bandwidth product, and the circuit feedback just like you can calculate the operational amplifier's output impedance *after* taking into account feedback. They are both the same calculation except with a virtual ground, the feedback impedance adds the output impedance of the operational amplifier.In practice as long as you stay within the output loading limits of the operational amplifier, virtual ground errors at DC are about as close to zero as you can get and errors at higher frequencies follow the amplifier's frequency response.

author: SArepairman
author_rank: Frequent Contributor
timestamp: April 26, 2014, 06:59:20 pm
content: well actually a digipot is useless for me, i realized that I have gain from 1-128... but its like an exponential curve. 20-60-80-120... not very useful. I mistakenly thought it would be a bit more linear from 0-128

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2014, 07:25:27 pm
content: Quote from: SArepairman on April 26, 2014, 06:59:20 pmAren't there separate linear and logarithmic versions intended for audio level control?