AD7991 Analog Devices, AD7991 Datasheet - Page 19

• Manufacturer Part Number: AD7991
• Description: 4-Channel, 12-Bit ADC with I2C Compatible Interface in 8-Lead SOT-23
• Manufacturer: Analog Devices

Specifications of AD7991

• Resolution (bits): 12bit
• # Chan: 4
• Sample Rate: 140kSPS
• Interface: I²C/Ser 2-Wire
• Analog Input Type: SE-Uni
• Ain Range: Uni (Vref),Uni Vdd
• Adc Architecture: SAR
• Pkg Type: SOT

For ac applications, removing high frequency components from
the analog input signal is recommended by use of an RC band-
pass filter on the relevant analog input pin. 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. This 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. THD increases as the source impedance increases and
performance degrades. Figure 23 shows the THD vs. the analog
input signal frequency for different source impedances at a
supply voltage of 5 V.
Rev. B | Page 19 of 28
Figure 23. THD vs. Analog Input Frequency for Various Source Impedances
–100
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
1
for V
DD
= 5 V, f
ANALOG INPUT FREQUENCY (kHz)
SCL
= 1.7 MHz Without Clock Stretching
AD7991/AD7995/AD7999
10
V
V
TEMPERATURE = T
f
SCL
DD
REF
5.1kΩ
2kΩ
1.3kΩ
240Ω
56Ω
= 5V
= 1.7MHz
= V
DD
A
100