Datasheet

AD7401A
Rev. C | Page 14 of 20
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).
MODULATOR OUTPUT
+FS ANALOG INPUT
–FS ANALOG INPUT
ANALOG INPUT
07332-021
Figure 21. Analog Input vs. Modulator Output
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).
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 Voltage Input
Full-Scale Range +640 mV
Positive Full Scale +320 mV
Positive Typical Input Range +250 mV
Positive Specified Input Range +200 mV
Zero 0 mV
Negative Specified Input Range −200 mV
Negative Typical Input Range −250 mV
Negative Full Scale −320 mV
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.
65535
53248
SPECIFIED RANGE
ANALOG INPUT
ADC CODE
12288
–320mV –200mV +200mV +320mV
0
07332-022
Figure 22. Filtered and Decimated 16-Bit Transfer Characteristic
Σ-∆
MOD/
ENCODER
INPUT
CURRENT
NONISOL
A
TED
5V/3V
ISO
L
A
TED
5V
V
DD1
R
SHUNT
V
IN
+
V
IN
–
GND
1
V
DD
GND
V
DD2
MDAT MDAT
SINC3 FILTER*
AD7401A
MCLKIN
SDAT
CS
SCLK
MCLK
GND
2
DECODER
DECODER
+
ENCODER
07332-023
*THIS FILTER IS IMPLEMENTED
WITH AN FPGA OR DSP.
Figure 23. Typical Application Circuit