Datasheet

AD7357
Rev. B | Page 13 of 20
For ac applications, it is recommended to remove high frequency
components from the analog input signal by the use of an RC
low-pass filter on the analog input pins. 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 significantly affect the ac performance of the
ADC and 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 THD that can be tole-
rated. The THD increases as the source impedance increases
and performance degrades. Figure 17 shows a graph of the
THD vs. the analog input signal frequency for various source
impedances.
–89
–87
–85
–83
–81
–79
–77
–75
–73
–71
–69
–67
–
65
100 200 1000 1500 2000 2500
FREQUENCY (kHz)
THD (dB)
10Ω
33Ω
50Ω
100Ω
07757-017
Figure 17. THD vs. Analog Input Frequency for Various Source Impedances
Figure 18 shows a graph of the THD vs. the analog input
frequency while sampling at 4.2 MSPS. In this case, the
source impedance is 33 .
–90.0
–86.0
–82.0
–78.0
–74.0
–70.0
–
66.0
0 1000 2000 3000 4000 5000
ANALOG INPUT FREQUENCY (kHz)
THD (dB)
07757–118
Figure 18. THD vs. Analog Input Frequency
ANALOG INPUTS
Differential signals have some benefits over single-ended
signals, including noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance. Figure 19 defines the fully differential input
of the AD7357.
V
IN+
AD7357*
V
IN–
V
REF
p-p
V
REF
p-p
*
ADDITIONAL PINS OMITTED FOR CLARITY.
COMMON
MODE
V
OLTAGE
07757-034
Figure 19. Differential Input Definition
The amplitude of the differential signal is the difference
between the signals applied to the V
IN+
and V
IN−
pins in
each differential pair (V
IN+
− V
IN−
). V
IN+
and V
IN−
should be
simultaneously driven by two signals each of amplitude V
REF
that are 180° out of phase. This amplitude of the differential
signal is, therefore, –V
REF
to +V
REF
peak-to-peak regardless of
the common mode (CM).
CM is the average of the two signals and is, therefore, the
voltage on which the two inputs are centered.
CM = (V
IN+
+ V
IN−
)/2
This results in the span of each input being CM ± V
REF
/2. This
voltage has to be set up externally. When setting up the CM,
ensure that that V
IN+
and V
IN−
remain within GND/V
DD
. When
a conversion takes place, CM is rejected, resulting in a virtually
noise free signal of amplitude –V
REF
to +V
REF
corresponding to
the digital codes of 0 to 16,383.
DRIVING DIFFERENTIAL INPUTS
Differential operation requires V
IN+
and V
IN−
to be driven simulta-
neously with two equal signals that are 180° out of phase. Because
not all applications have a signal preconditioned for differential
operation, there is often a need to perform a single-ended-to-
differential conversion.
Differential Amplifier
An ideal method of applying differential drive to the AD7357
is to use a differential amplifier such as the AD8138. This part
can be used as a single-ended-to-differential amplifier or as a
differential-to-differential amplifier. The AD8138 also provides
common-mode level shifting. Figure 20 shows how the AD8138
can be used as a single-ended-to-differential amplifier. The
positive and negative outputs of the AD8138 are connected to
the respective inputs on the ADC via a pair of series resistors
to minimize the effects of switched capacitance on the front
end of the ADC. The architecture of the AD8138 results in
outputs that are very highly balanced over a wide frequency
range without requiring tightly matched external components.