Data acquisition
HOW TO LEVERAGE INPUT HIGH-Z TECHNOLOGY TO REDUCE SOLUTION POWER AND SIZE
Multiplexed (muxed) successive approximation register analogue-to-digital converter (SAR ADC) applications have size and power constraints, often dictated by the per-channel analogue signal chain design choices. This article describes why muxed SAR ADCs equipped with analogue input high-Z (high impedance) technology are key to obtaining a substantial reduction in solution size and power, without compromising performance and accuracy.
By Sanjay Rajasekhar, principal analogue design engineer, and Arvind Shankar, staff design evaluation engineer, both with Analog Devices
In optical communications applications, laser biasing could be monitored via optical power measurement, while EEG/ECG signals from electrodes could be monitored in VSM applications. These muxed applications have some common requirements:
M
There are many channels to monitor. Generally, the ADC sequences through all the channels.
The channel voltages are generally uncorrelated with each other.
Tight constraints exist on system-level footprint and power.
Some challenges arise because of these requirements. As the ADC finishes a conversion on one channel, the sampling capacitor within the ADC is charged to the voltage of the channel. If this voltage on the sampling capacitor is substantially different from the voltage of the channel next in sequence, then the signal chain must be designed to settle the sampling capacitor to the new voltage accurately, in the allowed acquisition time. Traditionally, the solution to this problem has been to use a wideband driver amplifier, coupled with an RC filter. A typical signal chain is shown in Figure 1. The sensor could output a voltage or a
current, and the sensor interface circuit could be an instrumentation amplifier or a trans- impedance amplifier, respectively. The capacitor is normally an NP0/C0G type, as other types can cause significant distortion. NP0 capacitors have high linearity but low density. The NP0 capacitor is also chosen to be much larger in value than the ADC internal sampling capacitor. It performs two critical functions:
Reduction of the kickback from the ADC sampling capacitor
Reduction of the wideband noise of the signal chain by filtering the noise beyond the required settling bandwidth
In the traditional signal chain, one is forced to use a driver amplifier and a large capacitor per channel. Each driver amplifier could consume anywhere between tenths of a mA to a few mA. Each capacitor, including clearances, could take up around 1mm2
Figure 1. A signal chain with a traditional muxed SAR ADC. 60
board area. Replicating this signal chain over many channels has significant adverse effects on the system footprint and power consumption. This constitutes one of the major problems in muxed SAR ADC applications today.
September 2022 Instrumentation Monthly
uxed SAR ADCs are commonly used in applications where there is a need to constantly monitor multiple critical
variables in the system.
of the
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