EVAL-AD7401EDZ Analog Devices Inc, EVAL-AD7401EDZ Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD7401EDZ
• Description: EVALUATION BOARD I.C.
• Manufacturer: Analog Devices Inc
• Series: iCoupler®r

Specifications of EVAL-AD7401EDZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 20M
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±320 mV
• Power (typ) @ Conditions: 100mW @ 20MSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 105°C
• Utilized Ic / Part: AD7401
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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).
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.
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
AD7401
ENCODER
DECODER
MOD/
Σ-∆
+FS ANALOG INPUT
–FS ANALOG INPUT
Figure 23. Typical Application Circuit
Rev. C | Page 13 of 20
DECODER
ENCODER
MCLKIN
MDAT
GND
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
Full-Scale Range
Positive Full-Scale
Positive Specified Input Range
Zero
Negative Specified Input Range
Negative Full-Scale
To reconstruct the original information, this output needs to be
digitally filtered and decimated. A Sinc
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.
V
DD2
2
NONISOLATED
Figure 22. Filtered and Decimated 16-Bit Transfer Characteristic
5V/3V
65535
53248
12288
0
–320mV
SPECIFIED RANGE
V
MDAT
MCLK
GND
DD
SINC
3
FILTER
–200mV
ANALOG INPUT
3
CS
SCLK
SDAT
filter is recommended
+200mV +320mV
Voltage Input
+640 mV
+320 mV
+200 mV
0 mV
−200 mV
−320 mV
AD7401