AD7477AARM データシートの表示（PDF） - Analog Devices
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AD7477AARM Datasheet PDF : 24 Pages
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TYPICAL CONNECTION DIAGRAM
Figure 7 shows a typical connection diagram for the AD7476A/
AD7477A/AD7478A. VREF is taken internally from VDD and, as
such, VDD should be well decoupled. This provides an analog
input range of 0 V to VDD. The conversion result is output in a
16-bit word with four leading zeros followed by the MSB of the
12-bit, 10-bit, or 8-bit result. The 10-bit result from the AD7477A
will be followed by two trailing zeros, and the 8-bit result from
the AD7478A will be followed by four trailing zeros.
Alternatively, because the supply current required by the AD7476A/
AD7477A/AD7478A is so low, a precision reference can be used
as the supply source to the AD7476A/AD7477A/AD7478A. A
REF19x voltage reference (REF195 for 5 V or REF193 for 3 V)
can be used to supply the required voltage to the ADC (see Figure 7).
This configuration is especially useful if the power supply is
quite noisy or if the system supply voltages are at some value
other than 5 V or 3 V (e.g., 15 V). The REF19x will output a
steady voltage to the AD7476A/AD7477A/AD7478A. If the low
dropout REF193 is used, the current it needs to supply to the
AD7476A/AD7477A/AD7478A is typically 1.2 mA. When
the ADC is converting at a rate of 1 MSPS, the REF193 will
need to supply a maximum of 1.7 mA to the AD7476A/
AD7477A/AD7478A. The load regulation of the REF193 is typi-
cally 10 ppm/mA (VS = 5 V), which results in an error of 17 ppm
(51 µV) for the 1.7 mA drawn from it. This corresponds to a
0.069 LSB error for the AD7476A with VDD = 3 V from the
REF193, a 0.017 LSB error for the AD7477A, and a 0.0043 LSB
error for the AD7478A. For applications where power consumption
is of concern, the power-down mode of the ADC and the sleep
mode of the REF19x reference should be used to improve
power performance. See the Modes of Operation section.
1.2mA 0.1F
3V
1F
TANT
REF193
10F
0.1F
5V
SUPPLY
680nF
0V TO VDD
INPUT
VDD
VIN AD7476A/ SCLK
AD7477A/ SDATA
AD7478A
GND
CS
C/P
SERIAL
INTERFACE
Figure 7. REF193 as Power Supply to AD7476A/
AD7477A/AD7478A
AD7476A/AD7477A/AD7478A
Table I provides some typical performance data with various
references used as a VDD source for a 100 kHz input tone at
room temperature under the same setup conditions.
Table I. AD7476A Typical Performance for Various
Voltage References IC
Reference Tied
to VDD
AD780 @ 3 V
REF193
AD780 @ 2.5 V
REF192
REF43
AD7476A SNR Performance
(dB)
72.65
72.35
72.5
72.2
72.6
Analog Input
Figure 8 shows an equivalent circuit of the analog input structure
of the AD7476A/AD7477A/AD7478A. The two diodes, D1 and
D2, provide ESD protection for the analog input. Care must be
taken to ensure that the analog input signal never exceeds the
supply rails by more than 300 mV. This will cause these diodes
to become forward-biased and start conducting current into the
substrate. The maximum current these diodes can conduct
without causing irreversible damage to the part is 10 mA. The
capacitor C1 in Figure 8 is typically about 6 pF and can primarily
be attributed to pin capacitance. The resistor R1 is a lumped
component made up of the on resistance of a switch. This resistor
is typically about 100 Ω. The capacitor C2 is the ADC sampling
capacitor and has a capacitance of 20 pF typically. For ac applica-
tions, removing high frequency components from the analog input
signal is recommended by use of a band-pass filter on the relevant
analog input pin. In applications where harmonic distortion and
signal-to-noise ratio are critical, the analog input should be driven
from a low impedance source. Large source impedances will
significantly affect the ac performance of the ADC. This may
necessitate the use of an input buffer amplifier. The choice of
the op amp will be a function of the particular application.
VDD
D1
C2
R1 20pF
VIN
C1
6pF
D2
CONVERSION PHASE – SWITCH OPEN
TRACK PHASE – SWITCH CLOSED
Figure 8. Equivalent Analog Input Circuit
REV. C
–15–
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