LTC6416

13

6416f

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 filter-

ing options, which are often desirable when the load is an

ADC. Lowpass or bandpass filters are easily implemented

with just a few external components. The LTC6416 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 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

inputs so that the output common mode voltage equals

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

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 figure 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.

Figure 1. Input Termination for Differential 50Ω Input Impedance Using a 1:1 Balun

3

4

24.9Ω 0.1μF

50Ω

24.9Ω

6416 F01

7

8

LTC6416

0.1μF

0.1μF

1:1

+

–