AD7401AYRWZ-RL Analog Devices Inc, AD7401AYRWZ-RL Datasheet - Page 14

• Manufacturer Part Number: AD7401AYRWZ-RL
• Description: Isolated 16-Bit Sigma -Delta Modula I.C.
• Manufacturer: Analog Devices Inc

Specifications of AD7401AYRWZ-RL

• Number Of Bits: 16
• Sampling Rate (per Second): 20M
• Data Interface: Serial
• Number Of Converters: 1
• Power Dissipation (max): 93.5mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.300", 7.50mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7401EBZ - BOARD EVALUATION FOR AD7401
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD7401A
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7401A isolated Σ-Δ modulator converts an analog input
signal into a high speed (20 MHz maximum), single-bit data
stream; the time average single-bit data from the modulators
is directly proportional to the input signal. Figure 23 shows a
typical application circuit where the AD7401A is used to provide
isolation between the analog input, a current sensing resistor,
and the digital output, which is then processed by a digital filter
to provide an N-bit word.
ANALOG INPUT
The differential analog input of the AD7401A is implemented
with a switched capacitor circuit. This circuit implements a
second-order modulator stage that digitizes the input signal
into a 1-bit output stream. The sample clock (MCLKIN)
provides the clock signal for the conversion process as well as
the output data-framing clock. This clock source is external
on the AD7401A. The analog input signal is continuously
sampled by the modulator and compared to an internal
voltage reference. A digital stream that accurately represents
the analog input over time appears at the output of the
converter (see Figure 21).
A differential signal of 0 V results (ideally) in a stream of alter-
nating 1s and 0s at the MDAT output pin. This output is high
50% of the time and low 50% of the time. A differential input of
200 mV produces a stream of 1s and 0s that are high 81.25% of
the time (for a +250 mV input, the output stream is high 89.06% of
the time). A differential input of −200 mV produces a stream of
1s and 0s that are high 18.75% of the time (for a −250 mV
input, the output stream is high 10.94% of the time).
ANALOG INPUT
MODULATOR OUTPUT
Figure 21. Analog Input vs. Modulator Output
CURRENT
INPUT
R
SHUNT
+
ISOLATED
5V
V
V
V
GND
DD1
IN
IN
+
–
1
AD7401A
ENCODER
DECODER
MOD/
Σ-∆
+FS ANALOG INPUT
–FS ANALOG INPUT
Figure 23. Typical Application Circuit
Rev. B | Page 14 of 20
DECODER
ENCODER
MCLKIN
A differential input of 320 mV results in a stream of, ideally, all
1s. This is the absolute full-scale range of the AD7401A, and
200 mV is the specified full-scale range, as shown in Table 9.
Table 9. Analog Input Range
Analog Input
Full-Scale Range
Positive Full Scale
Positive Typical Input Range
Positive Specified Input Range
Zero
Negative Specified Input Range
Negative Typical Input Range
Negative Full Scale
To reconstruct the original information, this output needs to be
digitally filtered and decimated. A sinc3 filter is recommended
because this is one order higher than that of the AD7401A modu-
lator. If a 256 decimation rate is used, the resulting 16-bit word
rate is 62.5 kHz, assuming a 16 MHz external clock frequency.
Figure 22 shows the transfer function of the AD7401A relative
to the 16-bit output.
MDAT
GND
V
DD2
2
Figure 22. Filtered and Decimated 16-Bit Transfer Characteristic
65535
53248
12288
NONISOLATED
0
–320mV
5V/3V
SPECIFIED RANGE
SINC3 FILTER*
V
MCLK
GND
MDAT
DD
*THIS FILTER IS IMPLEMENTED
WITH AN FPGA OR DSP.
–200mV
ANALOG INPUT
+200mV +320mV
CS
SCLK
SDAT
Voltage Input
+640 mV
+320 mV
+250 mV
+200 mV
0 mV
−200 mV
−250 mV
−320 mV