AD7356 Analog Devices, AD7356 Datasheet - Page 13

• Manufacturer Part Number: AD7356
• Description: Differential Input, Dual, Simultaneous Sampling, 5 MSPS, 12-Bit, SAR ADC
• Manufacturer: Analog Devices

Specifications of AD7356

• Resolution (bits): 12bit
• # Chan: 2
• Sample Rate: 5MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (Vref) p-p,2.048 V p-p
• Adc Architecture: SAR
• Pkg Type: SOP

For ac applications, removing high frequency components from
the analog input signal is recommended 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 perfor-
mance 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, limit
the source impedance to low values. The maximum source
impedance depends on the amount of THD that can be
tolerated. 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 different source
impedances.
Figure 18 shows a graph of the THD vs. the analog input
frequency while sampling at 5 MSPS. In this case, the source
impedance is 33 Ω.
Figure 17. THD vs. Analog Input Signal Frequency for Various Source
–66
–70
–74
–78
–82
–86
–90
–65
–67
–69
–71
–73
–75
–77
–79
–81
–83
–85
–87
100
0
1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
Figure 18. THD vs. Analog Input Frequency
200
ANALOG INPUT FREQUENCY (kHz)
100Ω
Impedances
FREQUENCY (kHz)
1000
50Ω
1500
33Ω
10Ω
2000
2500
Rev. A | Page 13 of 20
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
performance. Figure 19 defines the fully differential input
of the AD7356.
The amplitude of the differential signal is the difference
between the signals applied to the V
each 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
the common mode (CM).
CM 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 V
a conversion takes place, CM is rejected, resulting in a virtually
noise-free signal of amplitude, –V
to the digital codes of 0 to 4095 for the AD7356.
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 conversion.
Differential Amplifier
An ideal method of applying differential drive to the AD7356
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-MODE
*
ADDITIONAL PINS OMITTED FOR CLARITY.
VOLTAGE
IN+
IN+
Figure 19. Differential Input Definition
and V
+ V
IN−
REF
IN−
)/2
IN+
to +V
remain within GND/V
− V
V
V
REF
REF
IN−
REF
IN+
). V
p-p
p-p
REF
peak-to-peak regardless of
and V
IN+
IN+
to +V
and V
and V
IN−
V
V
REF
AD7356*
IN+
IN–
to be driven
IN−
, corresponding
IN−
pins in
DD
should be
. When
REF
AD7356
/2. This
REF
)