Microprocessor Interfacing - v1.05 - J R Smith 8.4 Delta - Sigma Differential amplifier
Vin VDAC
+ _
Comparator
Integrator
V1
V2
+ _
V3
bitstream output
1-bit DAC The output of the differential amplifier ,
V1 = Vin - VDAC
The integrator output V2 = sum of V1 over many cycles. V3 is the output of a comparator: if V2>=0
V3 = 1 else V3 = 0
VDAC = +1.00 if V3 = 1 or VDAC = -1.00 if V3 = 0 The circuit operates at a very fast clock rate. If the value in the integrator is positive the DAC output will be positive and this will be subtracted from Vin, reducing the output of the integrator. Similarly if V2 is negative, then VDAC will be negative and the output of the integrator will be increased. Consequently a stream of pulses is produced whose average value reflects the value of Vin. This then undergoes sophisticated filtering via a decimation filter to become the final serial output. Advantages: cheap, very precise (a one bit DAC has nothing to go out of calibration), can perform very sophisticated filtering to remove noise and interference. Disadvantages: The clock rate must be very much higher than any changes in Vin. These ADC are increasingly used for digital audio (20 bit resolution at 96 kHz) and very low frequency measurements (23 bits at 20 Hz).
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