LTC6400-8
9
64008f
BLOCK DIAGRAM
APPLICATIONS INFORMATION
Circuit Operation
The LTC6400-8 is a low noise and low distortion fully
differential op amp/ADC driver with:
• Operation from DC to 2.2GHz –3dB bandwidth
• Fixed gain of 2.5V/V (8dB)
• Differential input impedance 400Ω
• Differential output impedance 25Ω
• Differential impedance of output fi lter 100Ω
The LTC6400-8 is composed of a fully differential amplifi er
with on chip feedback and output common mode voltage
control circuitry. Differential gain and input impedance
are set by 200Ω/500Ω resistors in the feedback network.
Small output resistors of 12.5Ω improve the circuit stability
over various load conditions. They also provide a possible
external fi ltering option, which is often desirable when the
load is an ADC.
Filter resistors of 50Ω are available for additional fi ltering.
Lowpass/bandpass fi lters are easily implemented with just
a couple of external components. Moreover, they offer
single-ended 50Ω matching in wideband applications and
no external resistor is needed.
The LTC6400-8 is very fl exible in terms of I/O coupling.
It can be AC- or DC-coupled at the inputs, the outputs or
both. Due to the internal connection between input and
output, users are advised to keep input common mode
voltage between 1V and 1.8V for proper operation. If the
inputs are AC-coupled, the input common mode voltage
is automatically biased approximately 450mV above V
OCM
and thus no external circuitry is needed for bias. The
LTC6400-8 provides an output common mode voltage
set by V
OCM
, which allows driving ADC directly without
external components such as transformer or AC coupling
capacitors. The input signal can be either single-ended
or differential with only minor difference in distortion
performance.
Input Impedance and Matching
The differential input impedance of the LTC6400-8 is 400Ω.
Usually the differential inputs need to be terminated to a
lower value impedance, e.g. 50Ω, in order to provide an
impedance match for the source. Several choices are avail-
able. One approach is to use a differential shunt resistor
(Figure 1). Another approach is to employ a wideband
transformer and shunt resistor (Figure 2). Both methods
provide a wideband match. The termination resistor or
the transformer must be placed close to the input pins in
13
64008 BD
1
V
+
2
V
OCM
14
7
15
+OUT
+OUTF
–OUTF
–OUT
+IN
IN+ OUT–
IN– OUT+
+IN
–IN
–IN
516
R
G
200Ω
R
OUT
12.5Ω
R
F
500Ω
R
G
200Ω
2k
R
F
500Ω
6
4
V
–
3
V
+
12
V
–
11
ENABLE
9
V
–
10
V
+
COMMON
MODE CONTROL
BIAS CONTROL
8
R
OUT
12.5Ω
R
FILT
50Ω
R
FILT
50Ω
C
FILT
2.7pF
5.3pF
