Title: Figure 12.39 Analog-to-digital conversion.
1Figure 12.39 Analog-to-digital conversion.
2Figure 12.40 The DAC output is a staircase
approximation to the original signal. Filtering
removes the sharp corners. (Note In addition to
smoothing, the filter delays the signal. The
delay is not shown.)
37?
6?
5?
4?
3?
2?
1?
0?
Figure 12.49 Output versus input for a 3-bit
flash A/D converter
4DNL (differential nonlinearity) and INL (integral
nonlinearity)
5Figure 12.41 Circuit symbol for a
digital-to-analog converter.
6Figure 12.42 DACs can be implemented using a
weighted-resistance network. (Note If di 1,
the corresponding switch is to the right-hand
side. For di 0, the i th switch is to the
left-hand side.)
7Figure 12.43 An R -- 2R ladder network. The
resistance seen looking into each section is
2R. Thus, the reference current splits in half at
each node.
8Figure 12.44 An n-bit DAC based on the R2R
ladder network.
9Figure 12.45 Parallel, simultaneous, or flash
A/D conversion.
10Figure 12.51a Successive approximation ADC.
11Initially, all bits are set to 0 In step 1, the
control logic sets MSB to 1 and if the comparator
output is high, MSB is set back to 0, otherwise
MSB remains 1 The process is repeated for the
next bit. After n steps, the process is complete,
and the input to the DAC is the digital code for
the analog input.
12Oversampling A/D converters
E(n)Y(n)-X(n) is defined as quantization noise,
Y(n) is the quantized output and X(n) is the
input. E(n) is between (-?/2, ?/2) Where ? is the
quantization level. E(n) is typically
approximated as an independent uniformly
distributed white noise and its power spectral
density is k , fs is the sampling
frequency. Therefore, increase the fs relative to
the signal bandwidth will give higher resolution
than Nyquist sampling converters. Even further,
if oversampling is combined with noise shaping,
such as in a Sigma-Delta A/D converter, then the
resolution could be even higher.