High-Voltage, Low-current current sense amplifier - Page 1
Forum section: electronics / projects

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: October 14, 2024, 09:24:10 am
content: I have a photodiode circuit that is biased at 80Volts, now I want to measure, the current using a current sense resistor of 0.01 ohms, I was looking for a current sense amplifier IC that can take sense inputs like 80Volts, and can amplify currents of 50uA to a measurable level with a minimal noise.There are Either Voltage or low current ones, I can use discrete amplifier build one, I have very strict size requirements so an integrated solution can be better by both noise as well as size perspective.

author: moffy
author_rank: Super Contributor
timestamp: October 14, 2024, 09:37:19 am
content: Sorry, a bit confused, won't the APD feed into a current sense circuit like a TIA or just a resistor? Why would you need an extra current sense, especially one so small?

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: October 14, 2024, 10:00:18 am
content: I am not using a TIA, intead I am measuring the voltage drop if any light is incident, my APD can be biased to 78 volts but maximum current allowing is 200uA, So I want a safety mechanism from my Bias Source to protect my APD.Use the attached schematic for the reference.

author: moffy
author_rank: Super Contributor
timestamp: October 14, 2024, 10:07:04 am
content: You are already sensing the current across R3.

author: langwadt
author_rank: Super Contributor
timestamp: October 14, 2024, 10:11:43 am
content: makes no sense, why on earth do you want to measure with 0.01 Ohm?

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: October 14, 2024, 10:23:04 am
content: I want to make my APD very sensitive for that I have to bias it to very high Voltage, but I want to protect it as well if there is any increase in current due to any reason. However I am thinking to split R2 into 2 resistors in series and use one of them as current sense. I am current using a Keysight 34461A DMM and it measures the current. The current sensing will also help in health monitoring of APD, since I have readings of normal current readings at High Bias Voltage during long run drift in current values will tell about the health.

author: BennoG
author_rank: Frequent Contributor
timestamp: October 14, 2024, 01:26:19 pm
content: Quote from: Arsh ahmad on October 14, 2024, 10:23:04 amYou know the meter adds about 5in series to measure currents of 1mA and probably more on the lower ranges.Benno

author: langwadt
author_rank: Super Contributor
timestamp: October 14, 2024, 01:49:05 pm
content: Quote from: BennoG on October 14, 2024, 01:26:19 pmafaict specs say burden voltage on 0-1mA range is <0.11V, that's 110R

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: October 14, 2024, 05:52:17 pm
content: Quote from: BennoG on October 14, 2024, 01:26:19 pmYes, but my circuit anyway has a current limiting resistor of 120K so 100ohms will be nothing. I think what ever I mentioned in the initial question about 0.01 Ohm, that will not be required I can use 10K as well and use 110K series current limiting. But the thing is the voltage at resistor terminals will be in order of 70 volts, so I need a high voltage current sense amplifier that can accurate give a voltage output for uA that can be sensed by the ADC.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 14, 2024, 05:57:46 pm
content: I'm certainly no expert on the subject, and I clearly don't have as much experience as other experts do here, so all I can say is what I did in the past.If you're thinking of using a 0.01-ohm shunt resistor to measure current through the photodiode (assuming you're placing it between R3 and D1), if the current is small enough, the voltage drop across the shunt resistor would be small, wouldn't it? Would something like the INA293 or similar be enough?https://www.ti.com/lit/ds/symlink/ina293.pdf?ts=1728858797688&ref_url=https%253A%252F%252Fwww.google.com%252F

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 14, 2024, 07:17:04 pm
content: Quote from: Arsh ahmad on October 14, 2024, 10:00:18 amIf you want protection then surely you just need a simple < 200uA current source, not a 0.01 Ohm shunt resistor ?

author: Arsh ahmad
author_rank: Regular Contributor
timestamp: October 14, 2024, 07:45:49 pm
content: Quote from: PCB.Wiz on October 14, 2024, 07:17:04 pmBy the mechanism of a photodiode, If I am ramping up the voltage the current without any light may go form few Nano amps to upto few Micro amps, and to about some tenths of microamps with the incident light. What I want if my diode has some issues, while ramping up suppose at 5 volts it should consume few Nanoamps, but due to fault or unintentional short it may consume more, this system will help me to detect this slight unwanted increase in the voltage and hence will shutdown the voltage source before going to high voltage itself.

author: langwadt
author_rank: Super Contributor
timestamp: October 14, 2024, 07:48:43 pm
content: Quote from: Arsh ahmad on October 14, 2024, 07:45:49 pmyou are already measuring the current with R3

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 14, 2024, 07:52:07 pm
content: Quote from: Arsh ahmad on October 14, 2024, 07:45:49 pmA current limiter does all of that, automatically.It has low drop, until the current limit is reached.A PSSI2021SAY or NUD4011 is close to what you need, or you can roll your own.Addit : you can make a simple current mirror or current source using an LED and a BJT, or a BJT and two diodes.The LED+BJT can have better temperature stability, if that matters.I've added some Curves I found on this topic to this threadhttps://www.eevblog.com/forum/projects/led-as-a-zener-temperature-coeficient/or, you can make a current source using a LM4041-ADJ as per TI app notes and a HV PNP transistor.(TI's formula is slightly wrong, it gives closer to I(Vcc) than Iout. )