Low current source - Page 1
Forum section: electronics / projects

author: arivalagan13
author_rank: Contributor
timestamp: June 10, 2020, 03:29:15 pm
content: Hi all,I want to test a 10^9 transimpedance gain transimpedance amplifier. both for AC and DC.The range of currents I would like to test is between 100 pA to 0.1 nA.Any suggestions for doing the same, please?RegardsArM

author: graybeard
author_rank: Frequent Contributor
timestamp: June 10, 2020, 04:59:06 pm
content: Making current sources down to 1nA is easy. My typical go to circuit is this:I use a JFET for low currents <1µA and a BJT for larger. The issue you will have is leakage currents. You will need to use a FET input op-amp, but even so the input leakage currents from the ESD protection diodes may bite you since they are typically on the order of 10s to 100s of pA. You also need to be careful for the gate leakage current on the JFET.You may need to use your transimpedance amplifier to measure the input leakage current of op-amps.BTW how do you account for ESD protection diode leakage or input bias currents on your transimpedance amp?Did you know that 0.1nA = 100pA???

author: Benta
author_rank: Super Contributor
timestamp: June 10, 2020, 06:30:15 pm
content: Quote from: graybeard on June 10, 2020, 04:59:06 pmDid you ever try this in practice? The drain-source leakage current is in the uA range for a MOSFET, rising to almost the mA range when hot.BJTs are better, but still in the nA range.Unless of course you have access to some super-duper magic MOSFETs, JFETs and BJTs.

author: SiliconWizard
author_rank: Super Contributor
timestamp: June 10, 2020, 06:45:04 pm
content: Yes, I can't see how you could get this into the nA territory...

author: TimFox
author_rank: Super Contributor
timestamp: June 10, 2020, 09:05:00 pm
content: The Keithley "picoamp current sources", such as the 261http://web.mit.edu/8.13/8.13d/manuals/keithly-261-picoampere-source.pdfare nothing but an accurate voltage source and a collection of high-value resistors, equivalent to those used in the Keithley electrometers that a 261 would be used to calibrate or check. So long as the transimpedance amplifier has a negligible input voltage compared with the 261 voltage source, this is a simple solution. If there is a substantial series resistance in the amplifier's input protection network, it will change the calibration.

author: graybeard
author_rank: Frequent Contributor
timestamp: June 10, 2020, 09:26:07 pm
content: Did you ever try this in practice? The drain-source leakage current is in the uA range for a MOSFET, rising to almost the mA range when hot.BJTs are better, but still in the nA range.Unless of course you have access to some super-duper magic MOSFETs, JFETs and BJTs.I've had this work for one-of on test circuits. The lowest I have done for a production circuit is 100nA

author: Benta
author_rank: Super Contributor
timestamp: June 10, 2020, 09:34:35 pm
content: Quote from: graybeard on June 10, 2020, 09:26:07 pmThen please explain your contribution here. Why are you misleading people with obviously false statements?I limit myself to posts where I know what I'm talking about. In this case you've led a poster up the garden path.I'm sure you have a lot to contribute here, and we appreciate that. But stay on firm ground, please.

author: graybeard
author_rank: Frequent Contributor
timestamp: June 10, 2020, 10:24:11 pm
content: You can do many things for a one-of circuit when you can cherry pick each component that won't work for production. It seemed to me he was asking a one-of question.I said you need to pay attention to the leakage currents, because they are the limitation in this circuit. Perhaps you missed that.

author: Benta
author_rank: Super Contributor
timestamp: June 10, 2020, 11:11:30 pm
content: Quote from: graybeard on June 10, 2020, 10:24:11 pmNo I didn't. You mentioned this in the context of the op amp.Much more interesting is how to find a pass element that will go down to so low currents. I had the idea of a PN or PIN photodiode, but even thos have dark currents that are too high (~0.1 nA).It's an interesting problem.

author: duak
author_rank: Super Contributor
timestamp: June 11, 2020, 12:10:42 am
content: It has been a long time since I last looked but I think Keithley used a push-pull bipolar output stage and an electrometer grade amplifier to servo the current through the current sense resistor to the load. Leakage current in the output stage doesn't matter as long as the difference between the source and sink currents can be controlled by the current servo. I believe the Keithley 220 programable current source uses this circuit concept.If the desired current is limited to the pA/nA range and is unipolar it should be possible to add a current sink that can absorb the leakage current from pass element, whatever type it is. Note that the circuit must sense the current delivered to the load, not the current in the pass element.

author: arivalagan13
author_rank: Contributor
timestamp: June 11, 2020, 01:00:20 am
content: Thanks for pointing it out.In fact, the current range to be generated is between 10 pA to 100 pA (10 pA to 0.1 nA)

author: arivalagan13
author_rank: Contributor
timestamp: June 11, 2020, 01:44:50 am
content: Thank you all for the responses. I understand leakage current is a big concern.How about passing the ac signal (from AD9833 like IC's) through a capacitor? like the one discussed in a paper. I'm just copying and pasting it here (forgive me if I'm violating any rules here).It was taken from Circuit Design Considerations for Current Preamplifiers for Scanning Tunneling Microscopy by Alex Kandel"The preamplifier transimpedance gain was measured directly as a function of frequency, using a programmable sine-wave generator (AD9833, Analog Devices) at discrete frequencies between 20 Hz and 15 kHz. In order to measure noise levels, a 0.9 Hz square wave was used as a quasi-DC current source, and the frequency spectrum of the noise was obtained through the discrete-time Fourier transform. In both the cases, low-current test signals were generated by passing the output of a function generator through a capacitor. While a series resistor could also be used for this purpose, the highvalue resistor necessary to produce picoampere-to-nanoampere currents will typically have an undesirable level of parasitic capacitance and/or inductance.3,4 The 1.05 V AD9833 output was filtered, attenuated between 10 and 1000 times, and then passed through a 4 pF capacitor, producing sine-wave signals with amplitudes between 0.01 and 0.5 nA (depending on the frequency and attenuation). A 0.15 nA square wave was produced using a triangle-wave analog oscillator circuit based on a low-noise operational amplifier, also coupled through a 220 pF capacitor."If this can be done, they suggest the use of gas-filled capacitor...I'm unlucky, not able to find suitable commercial gas-filled capacitors.RegardaArM

author: Marco
author_rank: Super Contributor
timestamp: June 11, 2020, 04:41:37 am
content: You can certainly do that, but it's not really a current source. A modulated voltage connected through a resistor or capacitor will only produce a known current as long as the TIA can maintain the virtual ground.

author: mjs
author_rank: Regular Contributor
timestamp: June 11, 2020, 09:57:35 am
content: Do you need true DC indefinetely or would something like 1s be enough ?Use a voltage ramp (larger cap + current source) and a small low leakage (C0G is usually good, glass/PTFE) capacitor to turn dU/dt to current I=C*dU/dt.A small trace under the sensitive TIA input trace with a voltage ramp makes a nice way to implement a built in self test circuit. Ground carefully while operational, though.

author: welterde
author_rank: Contributor
timestamp: June 11, 2020, 09:01:43 pm
content: Quote from: Benta on June 10, 2020, 06:30:15 pmShouldn't something like 2N4392 (can be had for around 3EUR from mouser) work quite reasonably?Specified leakage at V_DS = 20V is 0.1nA max at ambient temperature.Couple that with one of those fA or pA input bias current opamps and you should be good to go, no?

author: Gyro
author_rank: Super Contributor
timestamp: June 11, 2020, 09:09:53 pm
content: QuoteLMC662, cheap and 3fA typ. at room temperature.

author: Benta
author_rank: Super Contributor
timestamp: June 11, 2020, 09:52:19 pm
content: Quote from: welterde on June 11, 2020, 09:01:43 pmSo a transistor that has a drain-source leakage current of 0.1 nA should be "reasonable" for a 10...100 pA current source?'Das glaube ich nicht, Tim...'

author: Marco
author_rank: Super Contributor
timestamp: June 12, 2020, 06:33:49 am
content: The options I see :Opamp based high compliance current source. TIA in reverse, chicken and egg probem ... can't test the current source at AC without a superior TIA and vice versaA modulated voltage source with a resistor (or capacitor). This is effectively a low compliance current source. The resistor or capacitor will have a lesser/different AC impedance than a photodiode or similar device, AC response will fall off differently. It's okay, but just be aware of the way it's different from the real deal.The real deal will be the proof in the pudding ... and unfortunately you'll have to optimize with imperfect information, no helping it.