AD7457 Analog Devices, AD7457 Datasheet - Page 12

• Manufacturer Part Number: AD7457
• Description: Pseudo Differential Input, 100 kSPS, 12-Bit ADC in 8-Lead SOT-23
• Manufacturer: Analog Devices

Specifications of AD7457

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 100kSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOT

AD7457
P-TO-P
ANALOG INPUT
The AD7457 has a pseudo differential analog input. The V
input is coupled to the signal source and should have an ampli-
tude of V
part. A dc input is applied to the V
input provides an offset from ground or a pseudo ground for
the V
V
main benefit of pseudo differential inputs is that they separate
the analog input signal ground from the ADC’s ground, allow-
ing dc common-mode voltages to be canceled.
Because the ADC operates from a single supply, it is necessary
to level shift ground-based bipolar signals to comply with the
input requirements. An op amp (for example, the AD8021) can
be configured to rescale and level shift a ground-based (bipolar)
signal, so that it is compatible with the input range of the
AD7457. See Figure 18.
When a conversion takes place, the pseudo ground corresponds
to 0 and the maximum analog input corresponds to 4096.
V
DD
REF
Figure 18. Op Amp Configuration to Level Shift a Bipolar Input Signal
+1.25V
–1.25V
to avoid exceeding the maximum ratings of the ADC. The
IN+
0V
input. Ensure that (V
REF
p-p to make use of the full dynamic range of the
DC INPUT
VOLTAGE
Figure 17. Typical Connection Diagram
V
IN
3R
R
R
EXTERNAL
V
0.33µF
REF
V
V
(2.5V)
R
0.33 µ F
IN+
IN–
V
V
IN−
REF
DD
0.1µF
AD7457
+ V
IN−
AD780
IN+
. The voltage applied to this
2.5V
1.25V
2.5V
) is less than or equal to
SDATA
0V
SCLK
GND
10µF
CS
V
V
IN+
IN
+2.7V TO +5.25V
SUPPLY
–
AD7457
INTERFACE
SERIAL
V
REF
µC/µP
IN+
Rev. A | Page 12 of 20
ANALOG INPUT STRUCTURE
Figure 19 shows the equivalent circuit of the analog input
structure of the AD7457. The four diodes provide ESD protec-
tion 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, which causes these diodes to become forward biased
and start conducting into the substrate. These diodes can con-
duct up to 10 mA without causing irreversible damage to the
part. Typically, the C1 capacitors in Figure 19 are 4 pF and can
be attributed primarily 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, which
typically have a capacitance of 16 pF.
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 the signal-to-noise ratio
are critical, the analog input should be driven from a low
impedance source. Large source impedances can significantly
affect the ac performance of the ADC, which 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 maxi-
mum source impedance depends on the amount of total
harmonic distortion that can be tolerated. The THD increases
as the source impedance increases and performance degrades.
Figure 20 shows a graph of the THD vs. analog input signal
frequency for different source impedances.
(Conversion Phase, Switches Open; Track Phase, Switches Closed)
V
V
IN+
IN
–
C1
C1
Figure 19. Equivalent Analog Input Circuit
D
D
D
D
V
V
DD
DD
R1
R1
C2
C2