Datasheet

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APPLICATION INFORMATION
THEORY OF OPERATION
ANALOG INPUT
INP
INM
25 W
25 W
RCR Filter
50 W
50 W
3.2 pF
C
1pF
par2
C
1pF
par2
L
3nH
pkg
L
3nH
pkg
C
2pF
bond
C
2pF
bond
R
200
esr
W
R
200
esr
W
C
0.8 pF
par1
Sampling
Switch
Sampling
Switch
Sampling
Capacitor
Sampling
Capacitor
R
15
on
W
R
15
on
W
R
10
on
W
C
4.0 pF
samp
C
4.0 pF
samp
S0237-01
ADS6424
ADS6423
ADS6422
SLAS532A – MAY 2007 – REVISED JUNE 2007
The ADS6425/ADS6424/ADS6423/ADS6422 (ADS642X) is a family of quad channel, 12-bit pipeline ADC up to
125 MSPS sampling frequency.
The conversion is initiated simultaneously by all the four channels at the rising edge of the external input clock.
After the input signals are captured by the sample and hold circuit of each channel, the samples are sequentially
converted by a series of low resolution stages. The stage outputs are combined in a digital correction logic block
to form the final 12-bit word with a latency of 12 clock cycles. The 12-bit word of each channel is serialized and
output as LVDS levels. In addition to the data streams, a bit clock and frame clock are also output. The frame
clock is aligned with the 12-bit word boundary.
The analog input consists of a switched-capacitor based differential sample and hold architecture, shown in
Figure 64 . This differential topology results in very good AC performance even for high input frequencies. The
INP and INM pins have to be externally biased around a common-mode voltage of 1.5 V, available on VCM pin
13. For a full-scale differential input, each input pin INP, INM has to swing symmetrically between VCM + 0.5 V
and VCM – 0.5 V, resulting in a 2-V
PP
differential input swing. The maximum swing is determined by the internal
reference voltages REFP (2.0V nominal) and REFM (1.0 V, nominal). The sampling circuit has a 3 dB bandwidth
that extends up to 500 MHz (see Figure 65 , shown by the transfer function from the analog input pins to the
voltage across the sampling capacitors).
Figure 64. Input Sampling Circuit
Copyright © 2007, Texas Instruments Incorporated 39
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