The high precision and high speed of an op amp directly affects the magnitude of power consumption. As the current consumption decreases, the gain bandwidth decreases; conversely, the offset voltage decreases and the current consumption increases.
Many of the electronic characteristics of an op amp interact and interact with each other. As the market demands for low-power applications such as wireless sensor nodes, Internet of Things (IoT) and building automation, understanding the balance between electronic features to ensure that both end device performance optimization and power consumption are as low as possible It is vital. This series of blog posts consists of three parts. In the first part, I will introduce the balance of power and performance of DC gain in nanopower precision op amps.
DC gain
You may recall the typical inversion of the op amp (as shown in Figure 1) and the non-inverted (Figure 2) gain configuration learned at school.
Figure 1: Inverting Operational Amplifier
Figure 2: Non-inverting op amp
According to these configurations, the closed-loop gain equations for the inverting and non-inverting op amps are obtained, Equation 1 and Equation 2:
In the equation, A_CL is the closed-loop gain, R_F is the feedback resistor value, and R_2 is the resistance value from the negative input to the signal (inverted) or ground (non-inverted).
These equations show that the DC gain is related to the resistance ratio and is independent of the resistance value. In addition, the "power" law and Ohm's law show the relationship between the resistance value and the power consumption (Equation 3):
P is the power consumed by the resistor, V is the voltage drop of the resistor, and I is the current flowing through the resistor.
For nanopower gain and voltage divider configurations, Equation 3 shows that the current consumption through the resistor is minimal, resulting in minimal power consumption. Equation 4 helps you understand the principle:
R is the resistance value.
Based on these equations, it can be seen that you must choose a large resistance value that provides both gain and power consumption (also called power consumption). If the current flowing through the feedback channel cannot be minimized, there is no advantage to using a nanopower op amp.
Once you have selected the resistor values ​​that meet your gain and power requirements, you also need to consider other electronic characteristics that affect the accuracy of the op amp's signal conditioning. Count the few systematic small errors inherent in non-ideal op amps and you will get the total offset voltage. Electronic Characteristics - V_OS is defined as the finite offset voltage between the op amp inputs and describes the error at a particular bias point. Please note that errors in all calculations are not recorded. To do this, gain error, bias current, voltage noise, common mode rejection ratio (CMRR), power supply rejection ratio (PSRR), and drift must be considered. This blog post does not provide a comprehensive discussion of all the parameters involved, and we will discuss in detail V_OS and drift, and their impact on nanopower applications.
In fact, op amps exhibit V_OS through the input, but sometimes it can be a problem in low frequency (approximate DC) precision signal conditioning applications. In the voltage gain section, as the signal is adjusted, the offset voltage will rise, producing a measurement error. In addition, the size of V_OS varies with time and temperature (drift). Therefore, low frequency applications require a fairly high resolution measurement method. It is important to choose a precision (V_OS ≤ 1mV) op amp with the lowest drift.
Equation 5 calculates the maximum temperature dependent V_OS:
I have already introduced the theoretical part, such as selecting a large resistance value for the low frequency application that can improve the gain ratio and the accuracy of the operational amplifier. Now I will use a two-lead electrochemical battery to give an example explanation. Two-lead electrochemical cells often emit low-frequency small signals and are used in a variety of portable sensing devices, such as gas detectors, blood glucose monitors, etc., to select a low-frequency (<10kHz) nanopower operational amplifier.
Using oxygen sensing (see Figure 3) as a specific application example, assuming the inductor's maximum output voltage is 10mV (converting the current to a voltage R_L through the manufacturer's specified load resistor), the op amp's full-scale output voltage is 1V. . With Equation 2, you can see that the value of A_CL needs to be 100, or R_F is 100 times that of R_2. Select a 100MΩ resistor and a 1MΩ resistor to get a gain of 101, and the resistor is large enough to limit current and minimize power consumption.
Figure 3: Oxygen sensor
To minimize offset error, the LPV821 zero-drift nanopower op amp is an ideal device. Using Equation 5 and assuming an operating temperature range of 0°C to 100°C, the maximum offset error produced by the device is:
Another ideal device is the LPV811 precision nanopower op amp. Collecting the necessary values ​​from its data table inserts Equation 5 to give:
(Note that the LPV811 data sheet does not specify the maximum upper limit of the offset voltage offset, so a typical value is used here).
If a general-purpose nanopower op amp is used instead, such as the TLV8541, the associated value change will result in:
(The TLV8541 data sheet does not specify the maximum upper limit of the offset voltage offset, so typical values ​​are still used here).
As you can see, the LPV821 op amp is ideal for this application. The LPV821 with a current consumption of 650nA can sense an oxygen sensor output voltage as low as 18μV or less with a maximum offset gain error of 2.3mV. If you need to meet both extreme precision and nanopower, a zero-offset nanopower op amp will be your best choice.
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