In the era of the Internet of Things, battery-powered applications are becoming more and more common. The challenge has always been to balance power savings with performance. However, modern op amps offer a solution that allows you to save up to 99% in power consumption without sacrificing accuracy. One way to achieve this is by using the disable pin.
The disable pin is typically used to put the amplifier into a low-power standby mode. In this state, the internal circuits consume very little power, making it ideal for applications where the amplifier isn't constantly needed. For example, if an op amp is used as a buffer for an ADC, it doesn’t need to be active all the time. It can be disabled when the ADC isn’t taking measurements, significantly reducing power usage.
Figure 1 shows a typical ADC input stage with an ADC driver and a reference buffer. The key is to determine when the amplifier must be active. The op amp needs to be turned on early enough before the ADC starts its conversion process to allow the input capacitor to charge properly. This charging time depends on factors like the capacitor size, the voltage being measured, and the op amp's ability to drive the load.
According to the AD7980 datasheet, the input capacitance is 30 pF when connected with 400 Ω impedance. However, op amps have different characteristics. The ADA4807, for instance, can handle higher capacitive loads, as shown in Figure 2. When disabled, the amplifier takes about 500 ns to reach full-scale output, which is around 5 V or 4.096 V.
To be safe, we assume the amplifier is turned on 750 ns before the start of the conversion. At 1 kSPS, this results in 99.83% power savings (only 0.02 mW), while at 1 MSPS, it’s 92.41% (10.75 mW). These numbers only account for the ADC driver; the reference buffer also benefits from the same power-saving strategy.
This example demonstrates how modern devices can achieve high performance with minimal power use. With a minimum sampling time of 500 ns, the SINAD deviation remains under 0.5 dB. For different configurations, such as inverting amplifiers, the power savings may vary, so it's important to test and analyze each case individually. By using faster, more efficient op amps, you can optimize both power and performance in your designs.
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