High-voltage Current Sensing ADC
February 23, 2015, ISSCC, San Francisco—Long Xu from Delft University described a current sensing ADC with +/- 30 V input common-mode input range. The likely applications include remote sensing, motor control, and current monitoring where weak differential signals are in a very high common-mode voltage setting.
The basic technology is to have a load at ground that converts the input into a current. This topology allows for wide common-mode input voltage and below ground common mode transients. The conventional solution is to have a wide common mode current sense amplifier in front of the ADC. Often, this is implemented as a chopper input, capacitive-coupled instrumentation amplifier.
The alternative for this work is to make a high common mode input range ADC to eliminate the front-end circuitry. Of the many possible architectures; delta-sigma, successive approximation, etc. feeding the switched capacitor circuits to block the large common-mode voltages. A typical switched capacitor integrator has issues with large offset, poor common-mode rejection ratio, and low input common-mode impedance. A simple modification of cross-coupled sampling cancels the input common mode voltage and doubles the input signal swing.
Previous versions of a high-voltage chopper suffered from very small input range of about 300 mVp-p. By cross-connecting the input switches you are note input limited by a more robust structure that can have input diodes to supply, ground, and on the inputs for protection. so the cross-coupled sampling allows for a +/- 30 V common-mode input voltage range and a 3-5 V operating range.
When a correlated double sampling and cross-coupled sampling are combined ahead of a delta-sigma ADC, the complex input stage reduces offset and 1/f noise in the amplifier. See figure 1 for phase and switch operation,
The overall performance of the design meets or exceeds all of the design requirements and are as follows:
Supply voltage: 5 V
Input common-mode range :+/- 30 V
Input differential-mode range: 4.4 Vp-p
Signal-noise ratio: 110 dB
Bandwidth: 100 Hz
Offset: 8 uV
Common-mode rejection ratio: 140 db
Gain accuracy: 0.6 %
Chip area: 0.8 mm2
Power: 505 uW
Figure of merit = snr + 10 log (bw/p): 163 dB
