AD7721ARZ-REEL Analog Devices Inc, AD7721ARZ-REEL Datasheet - Page 11

• Manufacturer Part Number: AD7721ARZ-REEL
• Description: 16-BIT SIGMA DELTA A-D I.C.
• Manufacturer: Analog Devices Inc

Specifications of AD7721ARZ-REEL

• Number Of Bits: 16
• Sampling Rate (per Second): 468.75k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 150mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-SOIC (0.300", 7.50mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

REV. A
CIRCUIT DESCRIPTION
Sigma-Delta ADC
The AD7721 ADC employs a sigma-delta conversion technique
that converts the analog input into a digital pulse train.
Due to the high oversampling rate, which spreads the quantiza-
tion noise from 0 to f
in the band of interest is reduced (Figure 8a). To reduce the
quantization noise further, a high order modulator is employed
to shape the noise spectrum, so that most of the noise energy is
shifted out of the band of interest (Figure 8b).
The digital filter that follows the modulator removes the large
out of band quantization noise (Figure 8c), while converting the
digital pulse train into parallel 12 bit wide binary data or serial
16 bit wide binary data.
Digital Filter
The digital filter that follows the modulator removes the large
out of band quantization noise, while converting the one bit
digital pulse train into 12-bit or 16-bit wide binary data. The
digital filter also reduces the data rate from f
the filter to f
rate is a little over twice the signal bandwidth which guarantees
that there is no loss of data in the signal band.
The AD7721 employs 2 FIR filters in series. The first filter is a
128 tap filter that samples the output of the modulator at f
The second filter is an 83 tap half-band filter that samples the
output of the first filter at f
frequency response of the 2 filters is shown in Figure 9.
Digital filtering has certain advantages over analog filtering.
First, since digital filtering occurs after the A/D conversion, it
can remove noise injected during the conversion process. Ana-
log filtering cannot do this. Second, the digital filter combines
low passband ripple with a steep roll off, while also maintaining
INTEREST
BAND OF
INTEREST
INTEREST
BAND OF
BAND OF
CLK
/32 at the output of the filter. The output data
Figure 8. Sigma-Delta ADC
QUANTIZATION NOISE
CLK
DIGITAL FILTER CUTOFF FREQUENCY
WHICH EQUALS 152.8kHz (10MHz) OR
229.2kHz (15MHz)
/2, the noise energy which is contained
SHAPING
NOISE
CLK
/16 and decimates by 2. The
a.
b.
c.
CLK
at the input of
f
CLK
f
f
CLK
CLK
/ 2
/ 2
/ 2
CLK
.
–11–
a linear phase response. This is very difficult to achieve with
analog filters.
Analog filters, however, can remove noise superimposed on the
signal before it reaches the ADC. Digital filtering cannot do this
and noise peaks riding on signals, near full-scale, have the po-
tential to overload the analog modulator even though the aver-
age value of the signal is within limits.
SERIAL INTERFACE
The AD7721’s serial communication port allows easy inter-
facing to industry-standard microprocessors, microcontrollers
and digital signal processors. The AD7721 is operated in self-
clocking mode, the AD7721 providing the serial clock. The
RFS signal is also provided by the AD7721 by tying RFS to
DRDY.
Figure 10 shows the timing diagram for reading from the
AD7721. DRDY goes high to indicate that a conversion has
been completed. DRDY remains high for one internal clock
(15 MHz) cycle and then goes low for the next 31 clock cycles.
New data is loaded into the output shift register on the rising
edge of DRDY. When DRDY goes low, the data is accessed
from the AD7721. Although the AD7721 has a 12-bit digital
output in the parallel mode, sixteen bits of data are available for
transmission in the serial mode, starting with the MSB. Serial
data is clocked out of the device on the rising edge of SCLK
and is valid on the falling edge of SCLK.
Figure 9a. 128 Tap FIR Filter Frequency Response
Figure 9b. 83 Tap FIR Filter Frequency Response
–100.0
–150.0
–50.0
–100.0
–150.0
0.0f
–50.0
0.0
0.0
0.0f
CLK
CLK
/32
0.2f
0.1f
CLK
CLK
/32
0.4f
0.2f
CLK
FREQUENCY
CLK
FREQUENCY
/32
0.6f
0.3f
CLK
CLK
/32
0.8f
0.4f
CLK
CLK
AD7721
/32
1.0f
0.5f
CLK
CLK
/32