EVAL-AD7400EDZ Analog Devices Inc, EVAL-AD7400EDZ Datasheet - Page 13

• Manufacturer Part Number: EVAL-AD7400EDZ
• Description: EVALUATION BOARD I.C.
• Manufacturer: Analog Devices Inc
• Series: iCoupler®r

Specifications of EVAL-AD7400EDZ

• Number Of Adc's: 1
• Number Of Bits: 16
• Sampling Rate (per Second): 10M
• Data Interface: Serial
• Inputs Per Adc: 1 Differential
• Input Range: ±320 mV
• Power (typ) @ Conditions: 90mW @ 10MSPS
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 105°C
• Utilized Ic / Part: AD7400
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7400 isolated Σ-Δ modulator converts an analog input
signal into a high speed (10 MHz typical), single-bit data
stream; the time average of the modulator’s single-bit data is
directly proportional to the input signal. Figure 22 shows a
typical application circuit where the AD7400 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 AD7400 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 (MCLKOUT)
provides the clock signal for the conversion process as well as
the output data-framing clock. This clock source is internal on
the AD7400. 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 20).
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 20. Analog Input vs. Modulator Output
CURRENT
INPUT
R
SHUNT
+
ISOLATED
5V
V
V
V
GND
DD1
IN
IN
+
–
1
ENCODER
DECODER
AD7400
MOD/
+FS ANALOG INPUT
–FS ANALOG INPUT
Σ-∆
Figure 22. Typical Application Circuit
Rev. D | Page 13 of 20
DECODER
ENCODER
MCLKOUT
A differential input of 320 mV results in a stream of, ideally, all
1s. This is the absolute full-scale range of the AD7400, 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 AD7400
modulator. If a 256 decimation rate is used, the resulting
16-bit word rate is 39 kHz, assuming a 10 MHz internal clock
frequency. Figure 21 shows the transfer function of the AD7400
relative to the 16-bit output.
MDAT
GND
V
DD2
2
NONISOLATED
Figure 21. Filtered and Decimated 16-Bit Transfer Characteristic
65535
53248
12288
5V/3V
0
–320mV
SPECIFIED RANGE
V
MDAT
MCLK
GND
DD
SINC
–200mV
3
FILTER
ANALOG INPUT
3
filter is recommended
+200mV +320mV
CS
SCLK
SDAT
Voltage Input
+640 mV
+320 mV
+200 mV
0 mV
−200 mV
−320 mV
AD7400