AD7403 Datasheet, PDF (16/25 Pages) Analog Devices – 16-Bit, Isolated Sigma-Delta Modulator

Data Sheet
AD7403
THEORY OF OPERATION
CIRCUIT INFORMATION
The AD7403 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 modulator is
directly proportional to the input signal. Figure 27 shows a
typical application circuit where the AD7403 is used to provide
isolation between the analog input, a current sensing resistor or
shunt, 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 AD7403 is implemented
with a switched capacitor circuit. This circuit implements a
second-order modulator stage that digitizes the input signal
into a single-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 externally
supplied to the AD7403. 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 28).
A differential signal of 0 V ideally results 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
250 mV produces a stream of 1s and 0s that are high 89.06% of
the time. A differential input of −250 mV produces a stream of
1s and 0s that are high 10.94% of the time.
A differential input of 320 mV ideally results in a stream of all
1s. A differential input of −320 mV ideally results in a stream of
all 0s. The absolute full-scale range is ±320 mV and the specified
full-scale performance range is ±250 mV, as shown in Table 12.
Table 12. Analog Input Range
Analog Input
Positive Full-Scale Value
Positive Specified Performance Input
Zero
Negative Specified Performance Input
Negative Full-Scale Value
Voltage Input (mV)
+320
+250
0
−250
−320
FLOATING
POWER SUPPLY
+400V
MOTOR
R_SHUNT
GATED
DRIVE
CIRCUIT
5.1V
220pF
10Ω
10Ω
FLOATING
POWER SUPPLY
220pF
GATED
DRIVE
CIRCUIT
10µF 1nF
V_DD1
GND₁
V_IN+
V_IN–
10µF 1nF
V_DD1
GND₁
AD7403
Σ-Δ
MOD/
ENCODER
DECODER
NONISOLATED
5V/3V
V_DD2
MDAT
DECODER
MCLKIN
V_DD
SINC3 FILTER*
MDAT
MCLK
ENCODER
GND₂
100nF
GND
CS
SCLK
SDAT
*THIS FILTER IS IMPLEMENTED
WITH AN FPGA OR DSP
–400V
Figure 27. Typical Application Circuit
MODULATOR OUTPUT
+FS ANALOG INPUT
–FS ANALOG INPUT
ANALOG INPUT
Figure 28. Analog Input vs. Modulator Output
Rev. B | Page 15 of 24