How To Minimize Sensing Measurement ErrorJuly 13, 2018 by Chris Anderson
In this how-to video, we’ll discuss the importance of op-amp noise performance in sensing applications.
In this video, we discuss the importance of op-amp noise performance in sensing applications using ROHM's LMR1802G-LB.
The LMR1802G-LB Vs. The BU7241
You might be working with an analog sensor for industrial equipment, or you might be monitoring current in a battery powered device, or you might need to amplify the output of a photodiode. All of these are sensing applications, and the noise performance of the op amp has a big impact on the measurement’s accuracy and precision. To show what kind of a difference a low noise op-amp makes in the real world, we have a demo of ROHM’s LMR1802G-LB.
On one side of the board, we have ROHM’s LMR1802G-LB, and on the other, we have a standard op-amp, in this case, it’s ROHM’s BU7241. While the standard op-amp is fine in many cases, this demo highlights the difference between it and the low-noise LMR1802G-LB.
Advantages of the LMR1802G-LB
The output will be 3.3 V ±2000 times the sum of the offset and noise. So ideally it would be 3.3 V. The green LED will stay lit until the noise exceeds certain thresholds, at which point the red then orange LEDs will light up. With that, I want to highlight two advantages provided by the LMR1802G-LB.
The first is noise. You can see the green LED stays lit for the LMR1802G-LB, while the standard op-amp is all over the place. We can also see the difference on the scope, with LMR1802G-LB being much tighter, with a delta of xx.x mV on its output, compared to the standard op amp, which has a much wider delta of xx.x mV. This is because the noise density of LMR1802G-LB is significantly lower than typical CMOS op-amps, and is actually similar to or better than that of bipolar op-amps: 2.9 nV/√Hz at 1 kHz and 7.8 nV/√Hz at 10 Hz.
The CMOS op-amp provides all the same benefits of CMOS: low input bias current, low supply voltage, and low power consumption. Another important factor in minimizing measurement error is the input offset voltage. As you can see, the standard op amp is showing aop-amput of approximately x.x V, but remember, ideally it would be 3.3 V. So you have a total offset of x.x V. The LMR1802G-LB, though has an offset of just x.x V. And remember, that offset is based on the input offset, which is multiplied by the gain, which, again, is 2000 in this case. For input offset, the LMR1802G-LB is spec’ed at 5 µV typical and 450 µV max. That’s about one fourth the input offset of a conventional op amp.
Besides the noise performance, I mentioned that the LMR1802G-LB being a CMOS op-amp provides some benefits over alternatives, such as bipolar op amps. Input bias current is rated at half a picoamp, it can operate from a 2.5 to 5.5 V single supply or ±1.25 to ±2.75 V dual supply, and it consumes about 1.1 mA of current. It also offers a best-in-class 68° phase margin and can drive capacitive loads up to 500 pF. To learn more about ROHM’s LMR1802G-LB, visit ROHM.com.
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