AD7451 Analog Devices, AD7451 Datasheet - Page 15

• Manufacturer Part Number: AD7451
• Description: Pseudo Differential Input, 1 MSPS, 12-Bit ADC in an 8-Lead SOT-23
• Manufacturer: Analog Devices

Specifications of AD7451

• Resolution (bits): 12bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOP,SOT

When no amplifier is used to drive the analog input, it is
recommended that the source impedance be limited to low
values. The maximum 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 25 shows a graph of THD vs. analog input signal
frequency for different source impedances.
Figure 26 shows a graph of THD vs. analog input frequency for
various supply voltages while sampling at 1 MSPS with an SCLK
of 18 MHz. In this case, the source impedance is 10 Ω.
Figure 25. THD vs. Analog Input Frequency for Various Source Impedances
Figure 26. THD vs. Analog Input Frequency for Various Supply Voltages
–100
–50
–55
–60
–65
–70
–75
–80
–85
–90
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
10k
10
T
T
V
A
A
DD
= 25°C
= 25°C
= 5V
V
DD
INPUT FREQUENCY (kHz)
INPUT FREQUENCY (Hz)
= 4.75V
V
V
DD
100Ω
DD
= 3.6V
= 2.7V
62Ω
100k
100
200Ω
10Ω
V
DD
= 5.25V
1000
1M
Rev. D | Page 15 of 24
DIGITAL INPUTS
The digital inputs applied to the AD7441/AD7451 are not limited
by the maximum ratings that limit the analog inputs. Instead,
the digital inputs applied, that is, CS and SCLK, can go to 7 V
and are not restricted by the V
input. The main advantage of the inputs not being restricted to
the V
avoided. If CS or SCLK are applied before V
of latch-up as there would be on the analog inputs if a signal
greater than 0.3 V were applied prior to V
REFERENCE
An external source is required to supply the reference to the
AD7441/AD7451. This reference input can range from 100 mV
to V
range 2.7 V to 5.25 V. The reference input chosen for an appli-
cation must never be greater than the power supply. Errors in
the reference source result in gain errors in the AD7441/AD7451
transfer function and add to the specified full-scale errors of the
part. A capacitor of at least 0.1 μF must be placed on the V
pin. Suitable reference sources for the AD7441/AD7451 include
the
tion diagram for the V
V
*ADDITIONAL PINS OMITTED FOR CLARITY.
DD
0.1µF
AD780
DD
DD
. The specified reference is 2.5 V for the power supply
+ 0.3 V limit is that power supply sequencing issues are
Figure 27. Typical V
10nF
and the ADR421. Figure 27 shows a typical connec-
0.1µF
NC
REF
NC = NO CONNECT
REF
1
2
3
4
pin.
V
TEMP
GND
Connection Diagram for V
IN
AD780
DD
OPSEL
V
TRIM
+ 0.3 V limits as on the analog
OUT
8
7
6
5
AD7441/AD7451
2.5V
DD
NC
NC
NC
DD
.
, there is no risk
DD
AD7451*
AD7441/
= 5 V
0.1µF
V
V
REF
DD
REF