ltc6416 Linear Technology Corporation, ltc6416 Datasheet - Page 13

• Manufacturer Part Number: ltc6416
• Description: 2 Ghz Low Noise Differential 16-bit Adc Buffer
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Circuit Operation
The LTC6416 is a low noise and low distortion fully dif-
ferential unity-gain ADC driver with operation from DC to
2GHz (–3dB bandwidth), a differential input impedance of
12kΩ, and a differential output impedance of 18Ω. The
LTC6416 is composed of a fully differential buffer with
output common mode voltage control circuitry and high
speed voltage-limiting clamps at the output. Small output
resistors of 9Ω improve the circuit stability over various
load conditions. They also simplify possible external fi lter-
ing options, which are often desirable when the load is an
ADC. Lowpass or bandpass fi lters are easily implemented
with just a few external components. The LTC6416 is very
fl exible in terms of I/O coupling. It can be AC- or DC-
coupled at the inputs, the outputs or both. When using
the LTC6416 with DC-coupled inputs, best performance is
obtained with an input common mode voltage between 1V
and 1.5V. For AC-coupled operation, the LTC6416 will take
the voltage applied to the V
inputs so that the output common mode voltage equals
V
has been designed to directly interface with the V
found on Linear Technology’s 16-, 14- and 12-bit high
speed ADC families.
Input Impedance and Matching
The LTC6416 has a high differential input impedance of
12kΩ. 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
CM
, thus no external circuitry is needed. The V
Figure 1. Input Termination for Differential 50Ω Input Impedance Using a 1:1 Balun
CM
V
pin and use it to bias the
IN
+
–
50Ω
0.1μF
0.1μF
CM
CM
1:1
pin
pin
24.9Ω 0.1μF
24.9Ω
matching using a 1:1 balun, while Figure 2 shows match-
ing using a 1:4 balun. 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 Figure 1, the capaci-
tor center-tapping the two 24.9Ω 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 LTC6416 for frequency
selection and/or noise reduction.
The noise performance of the LTC6416 also depends upon
the source impedance and termination. For example, the
input 1:4 balun in Figure 2 improves SNR by adding 6dB
of voltage gain at the inputs. A trade-off between gain
and noise is obvious when constant noise fi gure circle
and constant gain circle are plotted within the same input
Smith Chart. This technique can be used to determine
the optimal source impedance for a given gain and noise
requirement.
Output Match and Filter
The LTC6416 provides a source resistance of 9Ω at each
output. For testing purposes, Figure 3 and Figure 4 show
the LTC6416 driving a differential 400Ω load impedance
using a 1:1 or 1:4 balun, respectively.
The LTC6416 can drive an ADC directly without external
output impedance matching, but improved performance
can usually be obtained with the addition of a few external
components. Figure 5 shows a typical topology used for
driving the LTC2208 16-bit ADC.
3
4
IN
IN
LTC6416
+
–
OUT
OUT
6416 F01
+
–
8
7
LTC6416
13
6416f