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Chapter 6
Theory of Analog Operation
The DAC
Calibration
PCI-4451/4452/4453/4454 User Manual
The 64-times oversampling delta-sigma DACs on the PCI-4451/4453 work
in the same way as delta-sigma ADCs, only in reverse. The digital data first
passes through a digital lowpass filter and then goes to the delta-sigma
modulator.
In the ADC the delta-sigma modulator is analog circuitry that converts
high-resolution analog signals to high-rate, 1-bit digital data, whereas in
the DAC the delta-sigma modulator is digital circuitry that converts
high-resolution digital data to high-rate, 1-bit digital data. As in the ADC,
the modulator frequency-shapes the quantization noise so that almost all of
its energy is above the signal frequency. Refer to
information.
The digital 1-bit data is then sent directly to a simple 1-bit DAC. This
DAC can have only one of two analog values, and therefore is inherently
perfectly linear. The output of the DAC, however, has a large amount of
quantization noise at higher frequencies, and, as described in the
Anti-Image Filtering
section, some images still remain near multiples of
eight times the sample rate.
Two analog filters eliminate the quantization noise and the images. The
first is a fifth-order, switched-capacitor filter in which the cutoff frequency
scales with the sample frequency and is approximately 0.52 times the
sample frequency. This filter has a four-pole Butterworth response and
an extra pole at about 1.04 times the sample frequency.
The second filter is a continuous-time, second-order Butterworth filter
in which the cutoff frequency (at 80 kHz) does not scale with the sample
frequency. This filter mainly removes high-frequency images from the
64-times oversampled switched-capacitor filter. These filters cause
a delay between the input digital data and the output analog data of
34.6 ±0.5 sample periods.
The PCI-4451/4453 analog outputs have calibration adjustments. Onboard
calibration DACs remove the offset and gain errors for each channel.
For complete calibration instructions, refer to Chapter 5, Calibration.
6-14
The ADC
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