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AD7452 Datasheet PDF : 24 Pages
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Data Sheet
Figure 25 shows examples of the inputs to VIN+ and VIN– for
different values of VREF for VDD = 5 V. It also gives the maximum
and minimum common-mode voltages for each reference value
according to Figure 23.
REFERENCE = 2V
COMMON-MODE (CM)
CMMIN = 1V
CMMAX = 4V
VIN–
2V p-p
VIN+
REFERENCE = 2.5V
COMMON-MODE (CM)
CMMIN = 1.25V
CMMAX = 3.75V
VIN–
2.5V p-p
VIN+
Figure 25. Examples of the Analog Inputs to VIN+ and VIN– for
Different Values of VREF for VDD = 5 V
Analog Input Structure
Figure 26 shows the equivalent circuit of the analog input
structure of the AD7452. The four diodes provide ESD
protection for the analog inputs. Care must be taken to ensure
that the analog input signals never exceed the supply rails by
more than 300 mV. This causes these diodes to become
forward-biased and start conducting into the substrate. These
diodes can conduct up to 10 mA without causing irreversible
damage to the part. The capacitors, C1 in Figure 26, are
typically 4 pF and can primarily be attributed to pin
capacitance. The resistors are lumped components made up of
the on resistance of the switches. The value of these resistors is
typically about 100 Ω. The capacitors C2 are the ADC’s
sampling capacitors and have a typical capacitance of 16 pF.
VDD
D
VIN+
C1
D
R1
C2
VIN–
C1
VDD
D
D
R1
C2
Figure 26. Equivalent Analog Input Circuit
Conversion Phase—Switches Open; Track Phase—Switches Closed
AD7452
For ac applications, removing high frequency components from
the analog input signal through the use of an RC low-pass filter
on the relevant analog input pins is recommended. In applica-
tions where harmonic distortion and signal-to-noise ratio are
critical, the analog input should be driven from a low imped-
ance source. Large source impedances significantly affect the ac
performance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of the op amp is a function of
the particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases, and performance degrades.
Figure 27 shows a graph of the THD versus the analog input
signal frequency for different source impedances for VDD = 5 V.
0
TA = 25°C
VDD = 5V
–20
–40
RIN = 1kΩ
RIN = 510Ω
–60
–80
–100
10
RIN = 10Ω
RIN = 300Ω
100
277
INPUT FREQUENCY (kHz)
Figure 27. THD vs. Analog Input Frequency for Various
Source Impedances for VDD = 5 V
Figure 28 shows a graph of the THD vs. the analog input
frequency for VDD of 5 V ± 5% and 3 V + 20%/–10%, while
sampling at 555 kSPS with an SCLK of 10 MHz. In this case, the
source impedance is 10 Ω.
–50
TA = 25°C
–55
–60
–65
–70
–75
VDD = 3.6V
VDD = 2.7V
–80
–85
–90
10
VDD = 4.75V
100
VDD = 5.25V
277
INPUT FREQUENCY (kHz)
Figure 28. THD vs. Analog Input Frequency for 3 V and 5 V Supply Voltages
Rev. C | Page 15 of 24
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