AD7452 Analog Devices, AD7452 Datasheet - Page 15

• Manufacturer Part Number: AD7452
• Description: Differential Input, 555 kSPS, 12-Bit A/D Converter in 8-Lead SOT-23
• Manufacturer: Analog Devices

Specifications of AD7452

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 555kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOT

Figure 25 shows examples of the inputs to V
different values of V
and minimum common-mode voltages for each reference value
according to Figure 23.
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.
Conversion Phase—Switches Open; Track Phase—Switches Closed
V
V
IN+
IN–
Figure 25. Examples of the Analog Inputs to V
C1
C1
COMMON-MODE (CM)
COMMON-MODE (CM)
Figure 26. Equivalent Analog Input Circuit
Different Values of V
CM
CM
REF
CM
MAX
CM
MIN
for V
MAX
D
D
D
D
MIN
REFERENCE = 2.5V
= 1.25V
= 3.75V
REFERENCE = 2V
V
V
DD
DD
= 1V
= 4V
DD
= 5 V. It also gives the maximum
REF
V
V
V
V
for V
IN–
IN+
IN–
IN+
DD
2V p-p
2.5V p-p
= 5 V
IN+
R1
R1
IN+
and V
and V
C2
C2
IN–
IN–
for
for
Rev. B | Page 15 of 28
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 V
Figure 28 shows a graph of the THD versus the analog input
frequency for V
sampling at 555 kSPS with an SCLK of 10 MHz. In this case, the
source impedance is 10 Ω.
Figure 28. THD vs. Analog Input Frequency for 3 V and 5 V Supply Voltages
–100
–20
–40
–60
–80
–50
–55
–60
–65
–70
–75
–80
–85
–90
0
Figure 27. THD vs. Analog Input Frequency for Various
10
10
T
T
V
A
A
DD
= 25°C
= 25°C
DD
= 5V
of 5 V ± 5% and 3 V +20%/–10%, while
Source Impedances for V
INPUT FREQUENCY (kHz)
INPUT FREQUENCY (kHz)
R
IN
= 10Ω
R
V
IN
DD
= 510Ω
= 3.6V
V
DD
DD
R
= 5 V
= 4.75V
IN
100
100
= 1kΩ
V
DD
R
IN
= 2.7V
V
= 300Ω
DD
= 5.25V
AD7452
DD
277
277
= 5 V.