AD7357_08 AD [Analog Devices], AD7357_08 Datasheet - Page 13

• Manufacturer Part Number: AD7357_08
• Description: Differential Input, Dual, Simultaneous Sampling, 4.2 MSPS, 14-Bit, SAR ADC
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
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
tolerated. 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.
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 Ω.
ANALOG INPUTS
Differential signals have some benefits over single-ended
signals, including noise immunity based on the devices
common-mode rejection and improvements in distortion
Figure 17. THD vs. Analog Input Frequency for Various Source Impedances
Figure 18. THD vs. Analog Input Frequency
Rev. PrF | Page 13 of 20
performance. Figure 19 defines the fully differential input of the
AD7357.
The amplitude of the differential signal is the difference
between the signals applied to the V
differential pair (V
simultaneously driven by two signals each of amplitude V
that are 180° out of phase. This amplitude of the differential
signal is, therefore –V
common mode (CM).
The common mode is the average of the two signals and is
therefore the voltage on which the two inputs are centered.
This results in the span of each input being CM ± V
voltage has to be set up externally. When setting up the CM,
ensure that that V
conversion takes place, common mode is rejected resulting in a
virtually noise free signal of amplitude –V
corresponding to the digital codes of 0 to 16383 for the
AD7357.
DRIVING DIFFERENTIAL INPUTS
Differential operation requires V
simultaneously 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 conversions.
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.
CM = (V
COMMON-
VOLTAGE
*
ADDITIONAL PINS OMITTED FOR CLARITY.
MODE
IN+
Figure 19. Differential Input Definition
+ V
IN+
IN+
IN−
and V
− V
REF
)/2
to +V
IN−
V
V
IN−
REF
REF
). V
remain within GND/V
REF
p-p
p-p
IN+
IN+
peak-to-peak regardless of the
and V
and V
IN+
and V
V
V
IN−
AD7357*
IN+
IN–
REF
IN−
should be
to be driven
to +V
IN−
pins in each
REF
REF
AD7357
DD
/2. This
. When a
REF