AD7357BRUZ Analog Devices Inc, AD7357BRUZ Datasheet - Page 13

• Manufacturer Part Number: AD7357BRUZ
• Description: IC ADC DUAL14BIT 4.2MSPS 16TSSOP
• Manufacturer: Analog Devices Inc

Specifications of AD7357BRUZ

• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Design Resources: DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7357 (CN0061)
• Number Of Bits: 14
• Sampling Rate (per Second): 4.2M
• Number Of Converters: 2
• Power Dissipation (max): 59mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP (0.173", 4.40mm Width)
• Resolution (bits): 14bit
• Input Channel Type: Differential
• Supply Voltage Range - Analogue: 2.25V To 2.75V
• Supply Voltage Range - Digital: 2.25V To 3.6V
• Supply: RoHS Compliant
• Sampling Rate: 4.2MSPS
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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.
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.
Figure 17. THD vs. Analog Input Frequency for Various Source Impedances
–66.0
–70.0
–74.0
–78.0
–82.0
–86.0
–90.0
–65
–67
–69
–71
–73
–75
–77
–79
–81
–83
–85
–87
–89
100
0
Figure 18. THD vs. Analog Input Frequency
10Ω
33Ω
50Ω
100Ω
200
1000
ANALOG INPUT FREQUENCY (kHz)
FREQUENCY (kHz)
1000
2000
1500
3000
2000
4000
2500
5000
Rev. A | Page 13 of 20
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.
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 that V
a conversion takes place, CM is rejected, resulting in a virtually
noise free signal of amplitude –V
the digital codes of 0 to 16,383.
DRIVING DIFFERENTIAL INPUTS
Differential operation requires V
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.
CM = (V
VOLTAGE
COMMON
*
ADDITIONAL PINS OMITTED FOR CLARITY.
MODE
IN+
Figure 19. Differential Input Definition
+ V
IN+
IN−
and V
REF
)/2
IN+
to +V
− V
V
V
IN−
REF
REF
remain within GND/V
IN−
p-p
p-p
REF
IN+
REF
). V
and V
peak-to-peak regardless of
IN+
to +V
IN+
and V
and V
IN−
V
V
AD7357*
IN+
IN–
REF
to be driven simulta-
corresponding to
IN−
IN−
pins in
should be
REF
AD7357
DD
/2. This
. When
REF