AD7767 Analog Devices, AD7767 Datasheet - Page 22

• Manufacturer Part Number: AD7767
• Description: 24-Bit, 15 mW, 109 dB, 128 kSPS/64 kSPS/32 kSPS ADCs
• Manufacturer: Analog Devices

Specifications of AD7767

• Resolution (bits): 24bit
• # Chan: 1
• Sample Rate: 128kSPS
• Interface: Ser
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: SOP

AD7767
V
The AD7767/AD7767-1/AD7767-2 V
voltage in the range of 2.4 V to 2 × AV
recommended that the V
voltage reference. Examples of such references are the ADR445,
ADR435,
Typical reference supply circuits are shown in Figure 46.
The reference voltage input pin (V
supply to the AD7767/AD7767-1/AD7767-2 device. For a 5 V
V
be applied while voltage supplies to pins AV
This configuration reduces the number of different supplies
required.
The output of the low noise voltage reference does not require a
buffer; however, decoupling the output of the low noise reference is
important. Place a 0.1 μF capacitor at the output of the voltage
reference devices (ADR445, ADR435, ADR425, and ADR421)
and follow the decoupling advice provided for the reference
device chosen.
As mentioned, the nominal supply to the V
achieve the full dynamic range available. When a 2.5 V V
input is used (that is, in low power applications), the signal-to-
noise ratio and dynamic range figures (generated using a 5 V
V
6 dB, a direct result of halving the available input range.
The AD7767/AD7767-1/AD7767-2 device requires a 100 μF
capacitor to ground, which acts as a decoupling capacitor and as
a reservoir of charge for the V
close to the AD7767/AD7767-1/AD7767-2 device as possible.
Reducing the value of this capacitor (C40 in Figure 46) to 10 μF
typically degrades noise performance by 1 dB. C40 can be an
electrolytic or tantalum capacitor.
REFERENCE
REF+
REF+
REF+
SUPPLY
10µF
Figure 46. AD7767/AD7767-1/AD7767-2 Reference Input Configuration
C34
V+
input, a full-scale input of 5 V on both V
input) quoted in the Specifications section decrease by
INPUT SIGNAL
ADR425
C35
0.1µF
V
IN
(5 V output), and
ADR4xx
REF+
V
input be generated by a low noise
OUT
REF+
0.1µF
C39
pin. Place this capacitor as
REF+
ADR421
) also acts as a power
REF +
DD
(nominally 5 V). It is
C40
100µF
pin is supplied with a
REF+
DD
(2.5 V output).
IN+
V
remain at 2.5 V.
pin is 5 V to
REF+
and V
AD7767-1/
AD7767-2
AD7767/
IN−
REF+
can
Rev. C | Page 22 of 24
MULTIPLEXING ANALOG INPUT CHANNELS
The AD7767/AD7767-1/AD7767-2 can be used with a multi-
plexer configuration. As per any converter that uses a digital
filtering block, the maximum switching rate or the output data
rate per channel is a function of the digital filter settling time.
A user multiplexing the analog inputs to a converter that
employs a digital filter must wait the full digital filter settling
time before a valid conversion result can be achieved; after this
settling time, the channel can be switched. Then, the full
settling time must again be observed before a valid conversion
result is available and the input is switched once more.
The AD7767 filter settling time equals 74 divided by the output
data rate in use. The maximum switching frequency in a
multiplexed application is, therefore, 1/(74/ODR), where the
output data rate (ODR) is a function of the applied MCLK
frequency and the decimation rate employed by the device in
question. For example, applying a 1.024 MHz MCLK frequency
to the AD7767 results in a maximum output data rate of 128 kHz,
which in turn allows a 1.729 kHz multiplexer switching rate.
The AD7767-1 and the AD7767-2 employ digital filters with
longer settling time to achieve greater precision; thus, the
maximum switching frequency for these devices is 864 Hz and
432 Hz, respectively.