AD7827 Analog Devices, AD7827 Datasheet - Page 8

• Manufacturer Part Number: AD7827
• Description: 3/5V, 1 MSPS, 8-Bit, Serial Interface Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7827

• Resolution (bits): 8bit
• # Chan: 1
• Sample Rate: 1MSPS
• Interface: Ser
• Analog Input Type: SE-Uni
• Ain Range: Uni 2.0V,Uni 2.5V
• Adc Architecture: Pipelined
• Pkg Type: DIP,SOIC

AD7827
POWER-UP TIMES
The AD7827 has a 1 s power-up time when using an external
reference and a 25 s power-up time when using the on-chip
reference. When V
current mode of operation. In order to carry out a conversion
the AD7827 must first be powered up. The AD7827 is pow-
ered up by a rising edge on the CONVST pin and a conversion
is initiated on the falling edge of CONVST. Figure 9 shows
how to power up the AD7827 when V
after the ADC has been powered down using the CONVST pin
when using either the on-chip, or an external, reference. When
using an external reference the falling edge of CONVST may
occur before the required power-up time has elapsed; however,
the conversion will not be initiated on the falling edge of
CONVST but rather at the moment when the part has com-
pletely powered up, i.e., after 1 s. If the falling edge of
CONVST occurs after the required power-up time has elapsed,
it is upon this falling edge that a conversion is initiated. When
using the on-chip reference, it is necessary to wait the required
power-up time of approximately 25 s before initiating a con-
version, i.e., a falling edge on CONVST may not occur before
the required power-up time has elapsed, when V
nected or after the AD7827 has been powered down using the
CONVST pin as shown in Figure 9.
POWER VS. THROUGHPUT
Superior power performance can be achieved by using the
automatic power-down (Mode 2) at the end of a conversion
(see Operating Modes section of this data sheet).
Figure 10 shows how the automatic power-down is implemented
using the CONVST signal to achieve the optimum power per-
formance for the AD7827. The duration of the CONVST pulse
is set to be equal to or less than the power-up time of the de-
vices (see Operating Modes section). As the throughput rate is
reduced, the device remains in its power-down state for longer and
the average power consumption over time drops accordingly.
CONVST
CONVST
V
V
DD
DD
t
t
POWER-UP
CONVERSION INITIATED HERE
CONVERSION INITIATED HERE
POWER-UP
25 s
Figure 9. Power-Up Time
1 s
DD
is first connected, the AD7827 is in a low
EXTERNAL REFERENCE
ON-CHIP REFERENCE
DD
is first connected or
DD
is first con-
–8–
For example, if the AD7827 is operated in a continuous sam-
pling mode, with a throughput rate of 100 kSPS and using an
external reference, the power consumption is calculated as
follows. The power dissipation during normal operation is
30 mW, V
If the power-up time is 1 s and the conversion time is 330 ns
(@ 25 C), the AD7827 can be said to dissipate 30 mW for 1.33 s
(worst case) during each conversion cycle. If the throughput
rate is 100 kSPS, the cycle time is 10 s and the average power
dissipated during each cycle is (1.33/10) (30 mW) = 3.99 mW.
Figure 11 shows the Power vs. Throughput rate for automatic
full power-down.
CONVST
100
0.1
–10
–20
–30
–40
–50
–60
–70
–80
10
1
0
DD
0
0
t
POWER-UP
Figure 10. Automatic Power-Down
= 3 V.
Figure 11. Power vs. Throughput
1 s
50
50
100
Figure 12. AD7827 SNR
100
t
CONVERT
330ns
150
150
10 s @ 100kSPS
THROUGHPUT – kSPS
FREQUENCY – Hz
200
t
200
CYCLE
250
250
POWER-DOWN
300
300
350
350
2048 POINT FFT
SAMPLING
1MSPS
F
IN
= 30kHz
400
400
450
450
500
500
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