AD7621 Analog Devices, AD7621 Datasheet - Page 20

• Manufacturer Part Number: AD7621
• Description: 16-Bit, 2 LSB INL, 3 MSPS PulSAR® ADC
• Manufacturer: Analog Devices

Specifications of AD7621

• Resolution (bits): 16bit
• # Chan: 1
• Sample Rate: 3MSPS
• Interface: Byte,Par,Ser,SPI
• Analog Input Type: Diff-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

AD7621
Power-Up
At power-up, or returning to operational mode from the power-
down mode (PD = high), the AD7621 engages an initialization
process. During this time, the first 128 conversions should be
ignored or the RESET input could be pulsed to engage a faster
initialization process. Refer to the Digital Interface section for
RESET and timing details.
A simple power-on reset circuit, as shown in Figure 23, can be
used to minimize the digital interface. As OVDD powers up, the
capacitor is shorted and brings RESET high; it is then charged
returning RESET to low. However, this circuit only works when
powering up the AD7621 because the power down mode (PD =
high) does not power down any of the supplies. As a result,
RESET is low.
POWER DISSIPATION VS. THROUGHPUT
In impulse mode, the AD7621 automatically reduces its power
consumption at the end of each conversion phase. During the
acquisition phase, the operating currents are very low which
allows a significant power saving when the conversion rate is
reduced (see Figure 30). This feature makes the AD7621 ideal
for very low power, battery-operated applications.
It should be noted that the digital interface remains active even
during the acquisition phase. To reduce the operating digital
supply currents even further, drive the digital inputs close to the
power rails (that is, OVDD and OGND).
100k
10k
100
1k
100
IMPULSE MODE POWER
WARP MODE POWER
Figure 30. Power Dissipation vs. Sample Rate
1k
SAMPLING RATE (SPS)
10k
100k
PDREF = PDBUF = HIGH
1M
10M
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CONVERSION CONTROL
The AD7621 is controlled by the CNVST input. A falling edge
on CNVST is all that is necessary to initiate a conversion.
Detailed timing diagrams of the conversion process are shown
in Figure 31. Once initiated, it cannot be restarted or aborted,
even by the power-down input, PD, until the conversion is
complete. The CNVST signal operates independently of CS and
RD signals.
For optimal performance, the rising edge of CNVST should not
occur after the maximum CNVST low time, t
of conversion.
Although CNVST is a digital signal, it should be designed with
special care with fast, clean edges, and levels with minimum
overshoot, undershoot, or ringing.
The CNVST trace should be shielded with ground and a low
value (such as 50 Ω) serial resistor termination should be added
close to the output of the component that drives this line. Also,
a 60 pF capacitor is recommended to further reduce the effects
of overshoot and undershoot as shown in Figure 23.
For applications where SNR is critical, the CNVST signal should
have very low jitter. This can be achieved by using a dedicated
oscillator for CNVST generation, or by clocking CNVST with a
high frequency, low jitter clock, as shown in Figure 23.
CNVST
MODE
BUSY
ACQUIRE
t
t
3
5
t
Figure 31. Basic Conversion Timing
1
CONVERT
t
7
t
4
t
1
t
t
2
6
ACQUIRE
t
8
1
, or until the end
CONVERT