In the era of Internet of things, battery powered applications are booming. This article will show that we do not have to choose between power saving and accuracy. Some operational amplifiers have disable pins. If used properly, it can save up to 99% power consumption without affecting the accuracy. The disable pin is mainly used for static operation (standby mode). In this mode, all ICs are switched to a low-power state without using devices to process signals. This reduces power consumption by several orders of magnitude.
If the operational amplifier needs to be used as the buffer amplifier of ADC, as shown in Figure 1, it must be in working state to perform its function. However, if the amplifier is switched to off mode by disabling the pin, low power consumption can still be maintained. In general, the off mode can be used as long as the ADC does not need to read any new values into its sample and hold function block.
Figure 1 Typical schematic diagram of ADC input stage with ADC driver and reference voltage buffer.
The easiest way to implement this function is to convert the start command. In a standard ADC, the input (sample and hold) capacitor is first charged to the value to be measured. This part is completed before the signal is sent to the ADC for conversion. The input capacitor is then isolated and connected to the input of the converter stage, that is, the conversion begins. The conversion is then completed and the completed signal is set, depending on the converter type. Now the real question is: when must the operational amplifier be in operation? The amplifier must work long enough in advance of the conversion start signal to ensure that the internal input capacitance obtains the same value as the signal to be measured. The length of time depends on the size of the input capacitance, the voltage to be measured and the rate at which the operational amplifier drives the capacitive load.
The data book of ADC (ad7980) gives the input capacitance value of 30pf when 400 impedance is connected in series. However, operational amplifiers are not that simple. The capacitive load listed in the parameter table is 15pF, but it may be higher. See the corresponding curve (Figure 2). At the same time, it is necessary to consider the use of low-pass filter at 2.7nf and 20.
Figure 2 Frequency response of ada4807.
This graph shows that the module can drive a sufficiently high capacitive load. When disabled, the amplifier needs about 500ns to reach the full-scale output level. In this example, the maximum value is 5V or 4.096V.
For safety reasons, we assume that the amplifier is turned on 750 ns before the conversion starts. Compare the estimated data of 1ksps to 1msps.
At 1ksps, the power consumption may be saved by 99.83% (total power consumption of 0.02mw) and 92.41% (total power consumption of 10.75mw) at 1msps. This is only the power consumption saved by ADC driver; The reference voltage buffer can also save power consumption.
This example aims to illustrate the capabilities of modern devices. When the shortest sampling time is 500ns, the SINAD deviation is less than 0.5dB. For drivers, we also need to focus on faster related devices and use them flexibly. We only consider applications that are used as buffers (gain = 1). For inverting or other amplifiers, the power savings will also vary according to the specific situation. Further analysis by measurement is required.
16 bit resolution, no code loss
MSPs: swallowing rate 1
low power consumption
4 MW (1 MSPs, VDD only)
7 MW (1 MSPs, total power consumption)
seventy μ W (10 kSPS)
Inl: typical value ± 0.6 LSB, maximum value ± 1.25 LSB
SINAD: 91.25 dB (at 10 kHz)
Thd: – 110 dB (at 10 kHz)
Original title: how to balance low power consumption and high precision? Use good luck to put the “disable pin”
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Reviewed and edited by: Tang Zihong