Design Basics
The current being measured flows from +A1 to A1, developing a voltage across the shunt resistor (R34). This voltage is amplified by an instrumentation amplifier (U13, INA826) to produce an output voltage (A1_Out) that may be read by an analog to digital converter.
- A1_Out = (Voltage_across_shunt * Gain_of_amplifier) + Vref
- Voltage_across_shunt = Current_measured * Shunt_resistance
Gain
The data sheet for the amplifier specifies the Gain equation. You can use a set resistor (R35) to set the Gain to the desired value.
- In our amplifier, the Gain = 1 + (49400 / Rset).
- Rset = 510 Ohms, so the gain of the amplifier = 97.8
Reference voltage (Vref)
The reference voltage (REF, pin 6 of the amplifier) specifies the voltage the amplifier will output when there is 0 voltage applied to the amplifier input (0 current being measured).
When choosing REF, consider:
- The input voltage range of the device (ADC) you are using to measure the op amp output voltage (A1_Out). Is it capable of measuring both positive and negative (below GND) voltage or just positive voltage?
Hint: If you are using a computer connected USB DAQ, it is likely capable of reading both positive and negative, if you are using an ADC chip mounted on a circuit board it likely reads positive voltage only.
- Do you want to to measure current flowing in both directions or just one direction?
Current flowing in the positive direction (from +A1 to -A1) will generate a positive voltage output from the op amp. Current following in the opposite direction will generate a negative output from the op amp (below GND).
In our schematic, Vref = 0V.
Equation Describing Circuit
So the equation relating the measured current to the output voltage is:
- A1_Out = ((Current_measured * Shunt_resistance) * Gain) + Vref
- A1_Out = ((Current_measured * 332) * 97.8) + 0
For example, for 150uA the A1_Out voltage would be:
- (.00015 * 332) * 97.8 = A1_Out = 4.87v
Further Considerations
Measurement Resolution
How may bits (precision) is your ADC? Most of the cheap ADC chips offer 12 bits. USB DAQs aimed at the professional market use 14 or 16 bits.
The number of discrete levels your ADC will be able to translate the op amp output voltage (A1_Out) to is governed by the number of bits.
Discrete levels = 2 ^ ADC_bitness
- 12 bit ADC produces: 2¹² = 4096 discrete levels.
- A 14 bit ADC produces 16384 discrete levels.
Resolution example:
- Total current measurement range desired: -300uA to +300uA (600uA total range).
- 14 bit ADC (MagicDAQ) in use
- Maximum possible resolution: 600uA / 16384 levels = 0.036uA per level
- The actually resolution we achieve will be something less than the ideal resolution. You might observe that the ADC output ‘bounces’ between 1 or 2 levels. As such, let’s ‘generously round’ our ideal resolution figure to 0.1uA : a specification our system will comfortably deliver.
Given the bitness of your ADC, you must constrain the total current measurement range to achieve the desired output resolution.
‘Rail to Rail’ Instrumentation Amplifier
Select an instrumentation amplifier that specifies that it has ‘rail to rail’ output voltage. This means that the maximum output voltage the amp can produce is roughly equal to the supply voltage. You will want to design the maximum output voltage from the op amp (at maximum measured current) to be about 0.2V less than the rail supply voltage.
If you expect the op amp to produce negative voltage (below GND) you will need to select a ‘dual supply’ op amp and supply it with negative voltage.
Check out our post on charge pump DC to DC converters to see how you can easily produce a negative voltage supply rail.
Power Supply / Vref Supply Stability
The noise rejection on modern op amps is very good — you can use a switching regulator to produce the voltage that powers your instrumentation amplifier. Of course, a linear regulator is better — but it is not essential.
In contrast, it is critical that the Vref supply is free of noise. This voltage must be produced with a precision voltage reference, ideally located physically close to the instrumentation amplifier on the PCB. Of course, if Vref = GND, just connect the reference pin to the GND plane on your PCB.
Input Bias Current — Use an Instrumentation Amplifier!
Because we are measuring uAs, we need to consider the input bias current of the amplifier. There is a little current that flows into the measurement pins (pins 4 and 1, +IN and -IN on the amp). Instrumentation Amplifiers typically have very low input bias current — the data sheet for our op amp specifies 35nA. This amount of bias won’t affect the accuracy of our measurements much.
Most general purpose Op Amps, and general purpose current measurement integrated circuits, have input bias currents that are far too high to allow accurate measurement of uAs.
Shunt (current sense) Resistor Selection
To measure uAs, your current shunt should have a resistance value roughly in the 100s of Ohms or greater.
You don’t want to have a very low resistance shunt (say 5 Ohms), with a very large Op Amp Gain, because the shunt will produce very little voltage (5Ohms *150uA = only 750uV). At this level, the general noise on your PCB (‘noise floor’) will be similar in magnitude to the voltage you are trying to measure. Your amplifier will amplify both the noise and your signal — resulting in overall low accuracy.
Measuring Higher Currents (Amps) / Removing the Shunt From the Circuit
At higher currents (mAs and up), a large current measurement shunt will create an unacceptable voltage drop (volts) causing the downstream system to be inoperable.
You can remove the current shunt be placing a physical or solid state relay across the shunt. Close the relay when you want the shunt to be ‘removed’ from the circuit powering your downstream system.
Operating Input Range / Common Mode Voltage
In most systems, the current measurement shunt is placed directly after the power supply, before power is distributed to the system.
As such, the voltage applied to the sense pins (+IN and -IN) will be roughly equal to the supply voltage.
Choose an instrumentation amplifier that specifies that its operating input range (Vcm) meets your needs. For our amplifier (INA826), the allowable range is from -Supply to +Supply -1V.
You may need to provide the op amp with larger voltage supply rails (+10V and -10V for example) to meet your common mode needs.
Extra Notes
- DNP = Do Not Populate. In the above schematic, DNP populate components are not fitted on the PCB. These positions are left open for future flexibility in the design.
Need Some Help?
We’re always happy to share our knowledge — feel free to email us at:
- support@magicdaq.com
Working Model
Want to play with a working model? You can find this circuit on the M&A Board at MagicDAQ.com
