Datasheet

AD7401
Rev. D | Page 13 of 20
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7401 isolated Σ-Δ modulator converts an analog input
signal into a high speed (20 MHz maximum), single-bit data
stream; the time average of the modulator’s single-bit data is
directly proportional to the input signal. Figure 23 shows a
typical application circuit where the AD7401 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 AD7401 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 AD7401. 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
0
5851-020
Figure 21. Analog Input vs. Modulator Output
A differential signal of 0 V results (ideally) in a stream of 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 pro-
duces a stream of 1s and 0s that are high 81.25% of the time. A
differential input of −200 mV produces a stream of 1s and 0s
that are high 18.75% 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 AD7401, while
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 Specified Input Range +200 mV
Zero 0 mV
Negative Specified Input Range −200 mV
Negative Full-Scale −320 mV
To reconstruct the original information, this output needs to be
digitally filtered and decimated. A Sinc
3
filter is recommended
because this is one order higher than that of the AD7401 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 AD7401 relative to
the 16-bit output.
65535
53248
SPECIFIED RANGE
ANALOG INPUT
ADC CODE
12288
–320mV –200mV +200mV +320mV
0
0
5851-021
Figure 22. Filtered and Decimated 16-Bit Transfer Characteristic
Σ-∆
MOD/
ENCODER
INPUT
CURRENT
NONISOLATED
5V/3V
ISOLATED
5V
V
DD1
R
SHUNT
V
IN
+
V
IN
–
GND
1
V
DD
GND
V
DD2
MDAT MDAT
SINC
3
FILTER
AD7401
MCLKIN
SDAT
CS
SCLK
MCLK
GND
2
DECODER
DECODER
+
ENCODER
05851-019
Figure 23. Typical Application Circuit