AD7400AYRWZ-RL Analog Devices Inc, AD7400AYRWZ-RL Datasheet - Page 13

• Manufacturer Part Number: AD7400AYRWZ-RL
• Description: Isolated Sigma-Delta Modulator ADC I.C.
• Manufacturer: Analog Devices Inc
• Type: Modulatorr

Specifications of AD7400AYRWZ-RL

• Resolution (bits): 14 b
• Data Interface: Serial
• Voltage Supply Source: Single Supply
• Voltage - Supply: 4.5 V ~ 5.5 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.300", 7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Sampling Rate (per Second): -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7400A 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 single-bit data from the modulator is
directly proportional to the input signal. Figure 22 shows a
typical application circuit where the AD7400A 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 AD7400A 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 AD7400A. 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 ideally results 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 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 20. Analog Input vs. Modulator Output
CURRENT
INPUT
R
SHUNT
+
ISOLATED
5V
V
V
V
GND
DD1
IN
IN
+
–
1
AD7400A
ENCODER
DECODER
MOD/
Σ-Δ
+FS ANALOG INPUT
–FS ANALOG INPUT
Figure 22. Typical Application Circuit
DECODER
ENCODER
Rev. B | Page 13 of 20
MCLKOUT
MDAT
GND
A differential input of 320 mV ideally results in a stream of all
1s. This is the absolute full-scale range of the AD7400A, while
250 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 Sinc
mended because this is one order higher than that of the
AD7400A 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 AD7400A relative to the 16-bit output.
V
DD2
2
NONISOLATED
Figure 21. Filtered and Decimated 16-Bit Transfer Characteristic
5V/3V
65535
53248
12288
0
–320mV
SPECIFIED RANGE
* THIS FILTER IS IMPLEMENTED
WITH AN FPGA OR DSP.
V
MDAT
MCLK
GND
DD
SINC
3
–200mV
FILTER*
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
3
filter is recom-
AD7400A