PCM1760 Burr-Brown Corporation, PCM1760 Datasheet - Page 10

• Manufacturer Part Number: PCM1760
• Description: Multi-Bit Enhanced Noise Shaping 20-Bit ANALOG-TO-DIGITAL CONVERSION SYSTEM
• Manufacturer: Burr-Brown Corporation

THEORY OF OPERATION
MULTI-BIT ENHANCED NOISE SHAPING
A block diagram of a typical 1-bit delta-sigma modulator is
shown in Figure 4.
In Figure 4, the quantizer consists of a single bit which has
two possible states, either “0” or “1”. The input signal is
sampled at a much higher sample rate than the nyquist
sampling frequency. The quantizer output data stream is
digitally filtered for higher resolution nyquist data. The
theoretical SNR is determined by the number of the order of
the integrator and the oversampling rate.
There is a practical limit to increasing the numbers of order
of the integrator due to an inherent oscillation in the modu-
lator. There is also a limit to increasing the sample rate due
to the increase in jitter sensitivity associated with high clock
frequencies.
The PCM1760 utilizes a four-bit quantizer instead of the
conventional one-bit method. The quantizing noise of a four-
bit quantizer is 1/16 of the one-bit version. Using the four-
bit quantizer allows for a lesser order number of the integra-
tor and a lower oversampling rate to achieve similar perfor-
mance to that of a more complex one-bit system.
A block diagram of the PCM1760 modulator is shown in
Figure 6. The PCM1760 is a fourth-order integrator that
samples at 64x oversampling, and samples left and right
channel input signal simultaneously.
FIGURE 4. Single Stage 1-Bit Delta-Sigma.
FIGURE 6. Multi-bit Enhanced Noise Shaping.
FIGURE 5. Single Stage Multi-bit Delta-Sigma.
Input
Input
Input
+
®
2nd Order
Integrator
+
+
PCM1760P/U DF1760P/U
Integrator
Integrator
+
2nd Order
Integrator
Quantizer
nBit
ADC
DAC
z
nBit
–1
Output
Output
4Bits
4Bits
ADC
DAC
Output
10
FIGURE 7. Recommended Power Supply Connection and
LAYOUT PRECAUTIONS
Analog common and digital common of the PCM1760 are
not connected internally. These should be connected to-
gether with the common of the DF1760 as close to the unit
as possible, preferably to a large ground plane under the
PCM1760.
The use of a separate +5V supply is recommended for the
PCM1760 and DF1760, and to connect the common at one
point as described above. Low impedance analog and digital
commons returns are essential for better performance.
The power supplies should be bypassed with tantalum ca-
pacitors as close as possible to the units. See Figure 7 for
recommended common connections and power supplies
bypassing.
The DF1760 accepts the four-bit 64fs noise shaped data
stream from the PCM1760 and decimates to 1/16 with an
initial filter, and then decimates to 1fs 20-bit data using a 4x
oversampling filter.
The PCM1760 and DF1760 combination achieves a dy-
namic range of 108dB and SNR of 110dB even with a
single-ended input.
+5V
Power Supply
Common
Digital
NOTE: (1) Tantalum 3.3µF. (2) Ceramic 0.1µF.
Decoupling.
GND
Common
Analog
+5V
28
22
6
(1)
Power Supply
DF1760
(1)
(2)
GND
27
21
8
7
PCM1760
(1)
(1)
(1)
–5V
20
9
8