Custom sample and hold / gated integrator circuit - Page 1
Forum section: electronics / projects

author: electron_plumber
author_rank: Regular Contributor
timestamp: December 21, 2024, 04:22:35 pm
content: I need to measure very narrow/low duty cycle voltage pulses (~1V, ~50nSec on-time, ~1 uSec period). I've found a couple popular options for synchronous narrow-pulse sampling throughout literature -- namely gated integrators and lock-in amplifiers. I like the simplicity of the integrator option, so I'm going to prototype one. After a bit of simulation, I've realized the switch elements around the integration capacitor (for the "gate" and "reset" functionality) have to befast -- to reduce aperture uncertainty of integration windowlow parasitic capacitance -- to manage injected charge into the integration capacitorreasonable rdson -- something 10 ohms or less would be ideal as to not compete against integrator timing resistorlow leakage -- to reduce "droop" when in "hold" modeI've found it difficult to find something that really solves all of these constraints at these timescales. My first instinct was to try out fast power fets, but they inject way too much charge into the integrator during switch events. I've found a few CMOS switch options that seem to be designed for low-charge injection applications, but they're a little resistive and (perhaps more importantly) a little slow, so they result in more aperture uncertainty. The DG611 being the fastest below.ADG1236: 1pC charge injection, ~120 ohm on resistance, >100nSec switch timeADG1201: 5pC charge injection, ~65 ohm on resistance, ~300nSec switch timeDG611: 3-4pC, ~25 ohm on resistance, ~15 nSec switch timeAnyone worked on something similar? Any switch/topology recommendations that would solve at this timescale?

author: electron_plumber
author_rank: Regular Contributor
timestamp: December 21, 2024, 07:13:21 pm
content: Hmm, just discovered the SN74LVC1G3157 part family, which seems suitable!For 3.3V operation:On state resistance: 9 OhmCharge injection: 3 pCswitch enable time: 2.5 nSec

author: jwet
author_rank: Frequent Contributor
timestamp: December 21, 2024, 09:38:08 pm
content: Time to Analog Converter kind of capture this whole problem in one IC. There used in LIDAR to measure time of flight itc. The TI TDC7200 is a popular one- capture range 12 to 500 nS, resolution 35 ps. Analog devices and Maxim make some too that are used in ATE.

author: moffy
author_rank: Super Contributor
timestamp: December 21, 2024, 09:44:51 pm
content: 50ns for a gated integrator is pretty fast and would need good technique, I would be tempted to use a simple resistor into, for the circuit, a largish capacitor so that the voltage rise during charging is small compared to the input voltage, giving a reasonably linear integration. I would then use the dead time to resolve that voltage accurately. Just a thought.

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2024, 12:27:39 am
content: 4-diode sampling bridge - accept no substitutes. Since the required speed is so slow, use DC coupled switched current sources to drive the bridge.A fast operational amplifier with shutdown would also work and be a lot simpler.

author: electron_plumber
author_rank: Regular Contributor
timestamp: December 22, 2024, 02:10:20 pm
content: Quote from: jwet on December 21, 2024, 09:38:08 pmI suppose that works if you're trying to measure pulse width -- my pulse width is fixed and I'm trying to measure amplitude (which may or may not be flat during the pulse interval), hence the integration step.Quote from: moffy on December 21, 2024, 09:44:51 pmHmm, I think the complexity is mostly in the gate/reset switch elements, not the integration itself, which would still be necessary with an R-C circuit. Why do you think an R-C would be better than an op-amp integrator? It's essentially also an R-C (feedback cap, input resistor) but with a deterministic slew rate, etc.Quote from: David Hess on December 22, 2024, 12:27:39 amI'll take a look at the 4-diode sampling bridge -- thanks! At first glance, looks like a peak detector as opposed to a proper integrator, which might be okay if my pulses are "flat" (tbd).I've thought about using the enable signal on a fast op-amp. I'm using an OPA855 in the front-end (the king of uncompensated op amps as far as I'm concerned) which has a 15nSec turn-on time and 120nSec turn-off time. Might be simpler, but much slower than using a SN74LVC1G3157 as a "gate" switch.

author: David Hess
author_rank: Super Contributor
timestamp: December 22, 2024, 04:09:26 pm
content: Quote from: electron_plumber on December 22, 2024, 02:10:20 pmWith a 4-diode bridge, the output follows the input. It does not peak detect at all. It is equivalent to an FET switch but much higher performance.It is not entirely clear to me what you are trying to measure. Do you want to integrate the entire pulse? Were you going to turn on the switch before the pulse, integrate the pulse, and then turn off the switch?A 4-diode bridge or other switch will measure the pulse level at a specific point if the RC acquisition time is shorter than the aperture. If the RC acquisition time is longer than the aperture, which is the case with fast sampling, then an integrated measurement is made over the aperture time. The same applies to a FET switch but they are so much slower and have a lot more charge injection.QuoteIf you want to integrate the entire pulse, an alternative is to use a fast operational transconductance amplifier like the LT1228. It will produce an output current proportional to pulse height, and is controlled by its set current which controls its transconductance. Check out figure 7 on page 6 of Linear Technology application note 120 attached below which might be exactly what you are looking for.Since the output of an operational transconductance amplifier is a current, a capacitor to ground serves as the integrating element. The same thing could be done with a traditional operational integrator, but they are usually much slower because of their requirement for unity gain stability.

author: electron_plumber
author_rank: Regular Contributor
timestamp: December 22, 2024, 05:35:03 pm
content: Quote from: David Hess on December 22, 2024, 04:09:26 pmAh, I see now. I had to dig into the archives to find a summary of this circuit -- it's basically a current-controlled switch [see attachment 1, from Nonlinear-Circuits-Handbook-1976]. It's a neat idea. Assuming schottkys are used (to avoid reverse recovery), you'll still havesomecharge to displace in the junction capacitance during a switch transient. But it would be small (per 1/2CVfwd^2) and presumably get swept into the current source. I'll have to tinker with that in spice, thanks. The obvious downside is that this requires a lot more circuity than a simple CMOS switch.Quote from: David Hess on December 22, 2024, 04:09:26 pmYes, that's the idea. Each cycle, a pulse arrives of unknown magnitude and "flatness". I want to integrate the whole thing to get area under curve. I have an onboard clock that's synchronous to the signal pulse, from which I can pretty easily derive an edge for the "gate" and "reset" controls. I agree FET switches inject more charge -- but, I'm still not convinced something like the SN74LVC1G3157 wouldn't work.Quote from: David Hess on December 22, 2024, 04:09:26 pmGood suggestion, I'll take a look at this. I like the programmable gain feature of that chip.

author: moffy
author_rank: Super Contributor
timestamp: December 22, 2024, 09:27:12 pm
content: Quote from: electron_plumber on December 22, 2024, 02:10:20 pmThe problem with the opamp integrator is that the -ve input or virtual gnd will shift from gnd for a pulse input, rising and falling edges, very BW/Slew rate dependent, which will introduce an error into the integration, for shortish pulses that can be significant depending on the accuracy required.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2024, 01:47:48 am
content: Quote from: electron_plumber on December 22, 2024, 05:35:03 pmThe charge injection from the diodes tends to cancel out, and the diodes may be matched to improve this.QuoteTurning the output on and off amounts to fast switching a current into the adjustment pin, which is easy enough with a differential pair.Besides the Linear Technology LT1228, Texas Instruments has the OPA860/OPA861 fast operational transconductance amplifier which may be even faster.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2024, 02:07:17 am
content: Quote from: moffy on December 22, 2024, 09:27:12 pmIt is a worst case application for an operational amplifier. It has to be unity gain compensated for stability because the integration capacitor forces a gain of 1 at high frequencies, but unity gain compensation limits bandwidth and slew rate.We have pretty fast operational amplifiers now, and it is also possible to use a current feedback operational amplifier as an integrator by putting a resistor in series with the inverting input to tailor the frequency response.