AD7982 Analog Devices, AD7982 Datasheet - Page 15

• Manufacturer Part Number: AD7982
• Manufacturer: Analog Devices

Specifications of AD7982

• Resolution (bits): 18bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOP

SINGLE-TO-DIFFERENTIAL DRIVER
For applications using a single-ended analog signal, either
bipolar or unipolar, the ADA4941 single-ended-to-differential
driver allows for a differential input to the part. The schematic
is shown in Figure 26.
R1 and R2 set the attenuation ratio between the input range and
the ADC range (V
the desired input resistance, signal bandwidth, antialiasing, and
noise contribution. For example, for the ±10 V range with a 4 kΩ
impedance, R2 = 1 kΩ and R1 = 4 kΩ.
R3 and R4 set the common mode on the IN− input, and R5 and R6
set the common mode on the IN+ input of the ADC. The common
mode should be close to V
with a single supply, R3 = 8.45 kΩ, R4 = 11.8 kΩ, R5 = 10.5 kΩ,
and R6 = 9.76 kΩ.
VOLTAGE REFERENCE INPUT
The AD7982 voltage reference input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins, as explained in the Layout section.
When REF is driven by a very low impedance source (for
example, a reference buffer using the
a 10 μF (X5R, 0805 size) ceramic chip capacitor is appropriate
for optimum performance.
If an unbuffered reference voltage is used, the decoupling value
depends on the reference used. For instance, a 22 μF (X5R,
1206 size) ceramic chip capacitor is appropriate for optimum
performance using a low temperature drift
If desired, a reference decoupling capacitor with values as small
as 2.2 μF can be used with a minimal impact on performance,
especially DNL.
Regardless, there is no need for an additional lower value ceramic
decoupling capacitor (for example, 100 nF) between the REF
and GND pins.
±5V, ..
±10V,
100nF
100nF
R5
R3
R1
Figure 26. Single-Ended-to-Differential Driver Circuit
R6
R4
REF
IN
FB
REF
–0.2V
+5.2V
). R1, R2, and C
R2
C
F
ADA4941
REF
OUTN
OUTP
/2. For example, for the ±10 V range
20Ω
20Ω
F
AD8031
are chosen depending on
2.7nF
2.7nF
10µF
ADR43x
or the AD8605),
IN+
IN–
GND
REF
reference.
AD7982
+5V REF
+2.5V
VDD
Rev. A | Page 15 of 24
POWER SUPPLY
The AD7982 uses two power supply pins: a core supply (VDD) and
a digital input/output interface supply (VIO). VIO allows direct
interface with any logic between 1.8 V and 5.5 V. To reduce the
number of supplies needed, VIO and VDD can be tied together.
The AD7982 is independent of power supply sequencing between
VIO and VDD. Additionally, it is very insensitive to power supply
variations over a wide frequency range, as shown in Figure 27.
To ensure optimum performance, VDD should be roughly half
of REF, the voltage reference input. For example, if REF is 5.0 V,
VDD should be set to 2.5 V (±5%).
The AD7982 powers down automatically at the end of each
conversion phase; therefore, the power scales linearly with the
sampling rate. This makes the part ideal for low sampling rates
(even of a few hertz) and low battery-powered applications.
10.000
1.000
0.100
0.010
0.001
95
90
85
80
75
70
65
60
10000
1
Figure 28. Operating Currents vs. Sampling Rate
Figure 27. PSRR vs. Frequency
10
SAMPLING RATE (SPS)
FREQUENCY (kHz)
100000
I
I
I
VDD
REF
VIO
100
AD7982
1000000
1000