How to achieve precision measurements with a nanopower budget (1): DC gain of a nanopower op amp
Part 1: DC Gain of a Nanopower Op Amp
By Gen Vansteeg - December 6, 2017
The performance of an operational amplifier (op amp) is closely tied to its power consumption. High precision and high speed typically come at the cost of increased power usage. As current consumption decreases, the gain bandwidth tends to reduce, while offset voltage may increase. This trade-off is critical in low-power applications such as wireless sensor nodes, IoT devices, and building automation systems.
Understanding how different op amp characteristics interact is essential for optimizing both performance and power efficiency. In this three-part blog series, I’ll explore the balance between power and performance, focusing on DC gain in nanopower precision op amps.
What is DC Gain?
You might remember from your studies that op amps can be configured in inverting or non-inverting modes. These configurations determine the closed-loop gain, which is crucial for signal conditioning. The formulas for these gains are as follows:
In these equations, ACL represents the closed-loop gain, RF is the feedback resistor, and R2 is the input resistor. These equations show that DC gain depends on the ratio of resistors, not their absolute values.
Power consumption is directly related to resistance through Ohm’s Law, as shown in Equation 3:
For nanopower applications, using large resistors minimizes current flow and thus reduces power consumption. However, larger resistors can introduce other issues like noise and offset errors, so careful selection is needed.
Once you’ve chosen the right resistor values for gain and power, it's important to consider other factors that affect accuracy—like offset voltage (VOS), bias current, noise, and drift. These parameters all contribute to the overall error in the system.
VOS is especially important in low-frequency applications, where even small offsets can cause significant measurement errors. It also changes with temperature, which is known as drift. For low-frequency precision measurements, selecting an op amp with a low VOS and minimal drift is crucial.
To illustrate, let's take an example of an oxygen sensor. These sensors often produce very small signals, making them ideal candidates for nanopower op amps. Using a 100MΩ and 1MΩ resistor combination, we can achieve a gain of 101, which is sufficient for amplifying a 10mV signal to 1V.
Choosing the right op amp is key. The LPV821, for instance, offers zero-drift performance and low power consumption, making it ideal for this type of application. Its maximum offset voltage over a wide temperature range is well within acceptable limits for most precision systems.
On the other hand, a general-purpose nanopower op amp like the TLV8541 may not perform as well in terms of offset and drift, leading to higher errors in sensitive applications.
In conclusion, when designing for low-power, high-precision systems, it's essential to carefully balance gain, power, and accuracy. The right op amp can make all the difference in achieving reliable results without excessive power use.
Thank you for reading the first part of “How to Make Precision Measurements with Nanopower Operational Amplifiers.†In the next part, I’ll discuss how ultra-precision micropower op amps can improve current sensing applications.
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