LTC6601-1 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6601-1
Linear Technology
'LTC6601-1' PDF : 40 Pages View PDF
LTC6601-1
APPLICATIONS INFORMATION
INTERFACING THE LTC6601 TO ADC CONVERTERS
The LTC6601’s rail-to-rail differential output and adjustable
output common mode voltage make the LTC6601 ideal
for interfacing to low voltage, single supply, differential
input ADCs. The sampling process of ADCs creates a
sampling transient that is caused by the switching-in
of the ADC sampling capacitor. The switching-in of this
sampling capacitor momentarily “shorts” the output of the
amplifier as charge is transferred between amplifier and
sampling capacitor. The amplifier must recover and settle
from this load transient before this acquisition period has
ended, for a valid representation of the input signal. The
LTC6601 will settle much more quickly from these peri-
odic load impulses than it does from a 2V input step, but
it is a good idea to add an RC network after the outputs
of the LTC6601 to decouple the sampling transient of the
ADC (See Figure 6). The capacitance of the decoupling
network serves to provide the bulk of the charge during
the sampling process, while the two resistors of the filter
network are used to dampen and attenuate any transient
induced by the ADC. The ADC’s sampling bandwidth will
often be much greater than that of the LTC6601, so hav-
ing this discrete RC filter will give the additional benefit
of band limiting broadband output noise.
The selection of the RC time constant is trial and error
for a given ADC, but the following guidelines are recom-
mended. Choose an RC pole frequency greater than the
cutoff frequency of the LTC6601. 80MHz RC filters are
good for filtering broadband noise. Lower frequency RC
filters improve SNR at the expense of settling time. The
resistors in the decoupling network should be at least 25Ω.
Too much resistance in the decoupling network leaves
insufficient settling time and will create a voltage divider
between the dynamic input impedance of the ADC and the
decoupling resistors. Using insufficient resistance might
prevent proper dampening of the load transient caused by
the sampling process, and prolong the time required for
settling. In 16-bit applications, this will typically require
a minimum of 11 RC time constants. It is recommended
that the capacitor is chosen with low dielectric absorption
(such as a C0G multilayer ceramic capacitor).
20 19 18
LTC6601-1
17 16
1
+
2
VIN–
+
BIAS
3
–
4
5
67
8
9 10
VOUT–
15
R
C1
CONTROL
14
3V
0.1μF
1μF
13
C2
12
VOCM
10nF
VOUT+
11
R
C1
D15
AIN+
•
•
D0
AIN–
VCM
GND
1μF
2.2μF
66011 F06
t = R • (C1 + 2 • C2)
3.3V
1μF
Figure 6. Interfacing the LTC6601 to A/D Converters
66011f
21
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