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| arivalagan13:
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) |
| arivalagan13:
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. Regarda ArM |
| Marco:
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. |
| mjs:
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. |
| welterde:
--- Quote from: Benta on June 10, 2020, 06:30:15 pm --- --- Quote from: graybeard on June 10, 2020, 04:59:06 pm ---Making current sources down to 1nA is easy. My typical go to circuit is this: I use a MOSFET or JFET for low currents <1µA and a BJT for larger. The issue you will have is leakage currents. --- End quote --- 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. --- End quote --- Shouldn'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? |
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