Question
Sensor output is amplified so that signal lies between -3 and +3 V. System
voltage is 3 V, -3V available from a voltage converter.
Is it possible in some way to use the AD7790 as ADC in that case? What other
possibilities are there to convert the -3 to +3 V signal? Need a low-power
solution.
Answer
1. Use a Low-power op amp to shift and attenuate the input signal so that the
+/-3V signal range becomes a 1.5V +/-1.5V signal range.
Power the AD7790 from +3V supply (converter GND connected to system ground) and
use the AD7790 in unbuffered mode which gives a rail to rail input swing.
Disadvantages
=> external op amp adds power and noise.
You can also achieve a similar effect by simply using an external resistor
network (see our AD7707 and AD7732) to attenuate and level-shift the signal so
that it will fit into the ADCs input range when the ADC is powered from 0 to
+3V supplies.
2. Power the AD7790 from the +/-3V supplies (and level shift the digital
control signals so that they are referred to -3V instead of ground.
Then you can drive the AD7790 directly with a +/-3V signal and supply the
AD7790 with a +/-3V reference (if the supply is ow enough noise)
Otherwise you can attenuate the input signal to e.g. +/-2.5V which gives you
the possibility of supplying the reference from +/-2.5V voltge reference ICs.
And then use the converter in buffered mode (to get around the added input
impedance from the resistive divider.