Datasheet

LTC6417
18
6417f
applicaTions inForMaTion
Circuit Operation
The LTC6417 is a low noise and low distortion fully dif-
ferential unity gain ADC driver with a –3dB bandwidth
spanning DC to 1.6GHz, a differential input impedance of
18.5kΩ, and a differential output impedance of 3Ω. The
LTC6417 is composed of a fully differential buffer with
output common mode voltage control circuitry and high
speed voltage-limiting clamps at the output. Lowpass or
bandpass filters are easily implemented with just a few
external components. The LTC6417 is very flexible in
terms of I/O coupling. It can be AC- or DC-coupled at the
inputs, the outputs or both. When using the LTC6417 with
DC-coupled inputs, best performance is obtained with an
input common mode voltage between 1V and 1.5V. For
AC-coupled operation, the LTC6417 will take the voltage
applied to the V
CM
pin and use it to bias the inputs so
that the output common mode voltage equals V
CM
, thus
no external circuitry is needed. The V
CM
pin has been
designed to directly interface with the V
CM
pin found on
Linear Technology’s high speed ADC families.
Input Impedance and Matching
The LTC6417 has a high differential input impedance of
18.5kΩ. The differential inputs may need to be terminated
to a lower value impedance, e.g. 50Ω, in order to provide
an impedance match for the source. Figure 1 shows input
matching and single-ended to differential conversion using
a 1:1 balun, while Figure 2 shows a similar circuit using
a 1:4 balun to achieve an additional 6dB of voltage gain.
These circuits provide a wideband impedance match.
The balun and matching resistors must be placed close
to the input pins in order to minimize the rejection due to
input mismatch. In Figures 1 and 2, the capacitor center-
tapping the two input termination resistors improves high
frequency common mode rejection. As an alternative to
this wideband approach, a narrowband impedance match
can be used at the inputs of the LTC6417 for frequency
selection and/or noise reduction.
Figure 2. Input Termination for Differential 50Ω Input Impedance Using a 1:4 Balun
Figure 1. Input Termination for Differential 50Ω Input Impedance Using a 1:1 Balun
• •
V
IN
IN
+
IN
–
8
9
24.9Ω
0.1µF
50Ω
24.9Ω
OUT
–
6417 F01
OUT
+
18
19
LTC6417
0.1µF
0.1µF
0.1µF
1:1
T1
MABA-007159-000000
+
–
5
4
1
3
2
V
IN
T1
TCM4-19+
IN
+
IN
–
8
9
OUT
–
6417 F02
OUT
+
18
19
LTC6417
50Ω
0.1µF
100Ω
+
–
100Ω
0.1µF
0.1µF
0.1µF
4
6
3
1