AD7825 Analog Devices, AD7825 Datasheet - Page 14

• Manufacturer Part Number: AD7825
• Description: 3 V/5 V, 2 MSPS, 8-Bit, 1-/4-/8-Channel Sampling ADCs
• Manufacturer: Analog Devices

Specifications of AD7825

• Resolution (bits): 8bit
• # Chan: 4
• Sample Rate: 2MSPS
• Interface: Par
• Analog Input Type: SE-Uni
• Ain Range: 2 V p-p,2.5V p-p
• Adc Architecture: Pipelined
• Pkg Type: DIP,SOIC,SOP

AD7822/AD7825/AD7829
POWER-UP TIMES
The AD7822/AD7825/AD7829 have a 1 μs power-up time
when using an external reference and a 25 μs power-up time
when using the on-chip reference. When V
the AD7822/AD7825/AD7829 are in a low current mode of
operation. Ensure that the CONVST line is not floating when
V
rising, the part attempts to power up before V
and can enter an unknown state. To carry out a conversion, the
AD7822/AD7825/AD7829 must first be powered up. The
AD7829 is powered up by a rising edge on the CONVST pin,
and a conversion is initiated on the falling edge of CONVST .
Figure 19 shows how to power up the AD7829 when V
connected or after the AD7829 has been powered down using
the CONVST pin when using either the on-chip reference 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 is not initiated on the
falling edge of CONVST but rather at the moment when the
part has completely powered up, that is, after 1 μs. If the falling
edge of CONVST occurs after the required power-up time has
elapsed, then 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 μs before
initiating a conversion; that is, a falling edge on CONVST must
not occur before the required power-up time has elapsed, when
V
down using the CONVST pin, as shown in Figure 19.
DD
DD
is applied. If there is a glitch on CONVST while V
is first connected or after the AD7829 has been powered
50
48
46
44
42
40
38
0
2 .
Figure 18. SNR vs. Input Frequency on the AD7825
1
3
INPUT FREQUENCY (MHz)
4
5
f
SAMPLE
6
DD
DD
is first connected,
= 2MHz
has fully settled
8
DD
DD
1
0
is
is first
Rev. C | Page 14 of 28
CONVST
CONVST
Figure 20 shows how to power up the AD7822 or AD7825 when
V
using the PD pin or the CONVST pin, with either the on-chip
reference or an external reference. When the supplies are first
connected or after the part has been powered down by the PD
pin, only a rising edge on the PD pin causes the part to power
up. When the part has been powered down using the CONVST
pin, a rising edge on either the PD pin or the CONVST pin powers
the part up again.
As with the AD7829, when using an external reference with the
AD7822 or AD7825, the falling edge of CONVST may occur
before the required power-up time has elapsed. If this is the
case, the conversion is not initiated on the falling edge of CONVST ,
but rather at the moment when the part has powered up
completely, that is, after 1 μs. 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 μs before initiating a conversion; that
is, a falling edge on CONVST must not occur before the
required power-up time has elapsed, when supplies are first
connected to the AD7822 or AD7825, or when the ADCs have
been powered down using the PD pin or the CONVST pin, as
shown in Figure 20.
DD
V
V
is first connected or after the ADCs have been powered down,
DD
DD
t
t
POWER-UP
POWER-UP
25µs
Figure 19. AD7829 Power-Up Time
1µs
EXTERNAL REFERENCE
INITIATED HERE
ON-CHIP REFERENCE
INITIATED HERE
CONVERSION
CONVERSION