# Null detector with MCU

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Nov 27, 2014.

1. ### Vindhyachal Takniki Thread Starter Member

Nov 3, 2014
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Problem:
1. I have to make a null detector with MCU.
2. Input voltage maximum is 30V dc.
3. Current to detect is 1nA to 300mA.
4. Galvanometer resistor selected is 100ohm. As this is standard in many analog null detector I have. And it also limits the current through power supply in case two opposite are zero & other two are open. In this case entire 30V will appear across 100 ohm.
5. I have selected Max9918 for this purpose.
6. Current measured should be bidirectional.

My solution:
1. I have divided ranges into 5 parts, 1nA-500nA , 500nA - 1000nA , 1uA-500uA , 500uA-1000uA , 1mA-300mA.
2. I have 100ohm galvanometer resistance into two parts. Reason is described later.
3. Max9918 has sense range of 50mV full.
4. For 1nA-500nA, Rsense = 50+50ohm = 100ohm
So input range = 0.1uV - 50uV. Gain = 40000.
So R9 = 39K & R10 = 1K.

So ouput for +ve current is 2.5V - 4.5V
& output for -ve current is 0.5V-2.5V.

5. For 500nA-1000nA, Rsense = 50+50ohm = 100ohm
So input range = 50uV - 100uV. Gain = 20000.
So R9 = 19K & R10 = 1K.

So ouput for +ve current is 3.5V - 4.5V
& output for -ve current is 1.5V-2.5V.

6. For 1uA-500uA, Rsense = 50ohm
So input range = 100uV - 50mV. Gain = 40.
So R9 = 39 & R10 = 1.

So ouput for +ve current is 2.5V - 4.5V
& output for -ve current is 0.5V-2.5V.

7. For 500uA-1000uA, Rsense = 50ohm
So input range = 25mV - 50mV. Gain = 40.
So R9 = 39 & R10 = 1.

So ouput for +ve current is 3.5V - 4.5V
& output for -ve current is 1.5V-2.5V.

8. For 1mA-300mA, still working on it.
I will add a small sense resistor in series with two 50ohm resistors.

1. Is it right method to do.
I am starter in analog field. So don't know if it will work. Theoretically it seems.
practically I haven't included noise of max9918. I am reading the datsheet/opamp book to understand better.

2. I was looking for simualtor for Max9918. So that i check on simulator before actual circuit. I found this
http://www.maximintegrated.com/en/design/tools/modeling-simulation/pspice/MAX9918.FAM

But how to use it.

3. Any suggestions on it.

4. I have attached my datasheet & circuit.

5. Am I disobeying CMVR here.
On interent I read it is V1+V2/ 2 .

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