DC1076A - ANALOG DEVICES - Product.Network

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1076

FILTER DIPLEXER

DESCRIPTION

Demonstration circuit 1076 is a filter diplexer. This cir-

cuit facilitates bench evaluation of high speed analog to

digital converters in an environment where coaxial cable

length exists between the ADC input and any non-

absorptive terminations such as bandpass filters that

will reflect sampling impulses. Technically DC1076 is an

absorptive filter, not a true diplexer. True diplexers will

also work; the original development used a true diplexer,

hence the demo circuit name. However, a true diplexer

of equivalent performance requires more parts and odd

component values to achieve good crossover perform-

ance.

Design files for this circuit board are available. Call

the LTC factory.

PowerPath is a trademark of Linear Technology Corporation

Table 1.

Performance Summary (T

= 25°C)

PARAMETER CONDITION VALUE

LPF Bandwidth 3dB cutoff f > 150MHz

Impedance 50

Ω

Out of Band Return Loss f > f

3DB

> 20dB

OPERATING PRINCIPLES

When transmission line length exists between reflective

terminations and the input of a High speed ADC, the dis-

turbance caused by the sample switches opening and

closing reflect back and forth between the reflective ter-

mination and the ADC input. In a practical layout, this

transmission line length is very short and the reflections

damp out between sample clocks. In a bench evaluation

environment, external connectorized filters and cables

often must be introduced between signal sources and

the ADC demonstration circuit board. This can easily

introduce 100x more low loss transmission line length

between the reflective termination of the filter and the

ADC input. As a result, the sampling glitch disturbance

does not completely dissipate in the circuit losses be-

tween sampling clock edges. This results in additional

distortion products, particularly elevated harmonics in

the ADC output. The DC1076 is an absorptive filter

which when placed directly on the input to the ADC aids

in attenuating the sample glitches and reducing the dis-

tortion products. The schematic for DC1076 is shown in

Figure 1 and the layout in Figure 2, note that the filter is

actually < 1.00 inch square. The sampling glitch exhib-

its very fast edges and therefore most of the frequency

content is at higher harmonic frequencies. Figure 3

shows the filter response, S21, and input and output

return losses, S11 and S22 respectively of the DC1076

Filter Diplexer. Figure 4 shows the same parameters for

a commercial 70MHz band pass filter, out of band en-

ergy is completely reflected back to the source. Figure 5

shows the cascaded response of the commercial filter

with the DC1076. The combination provides the pass-

band attribute of the 70MHz BPF while the return loss

presented to the ADC (S22) is 20dB in the upper stop

band to beyond 1GHz. The source of course will still see

complete reflection of out of band energy as S11 is

≅

0dB outside the pass band of the 70MHz BPF.

QUICK START GUIDE FOR DEMONSTRATION CIRCUIT 1076

FILTER DIPLEXER

QUICK START PROCEDURE

Demonstration circuit 1076 is easy to connect to a Lin-

ear Technology High Speed ADC board. The filter is pro-

vided with a Male SMA output connector which will di-

rectly connect to the input of the ADC demonstration

circuit without any adaptors. The input is fitted with a

Female SMA connector as used on the ADC demonstra-

tion circuits and so it can be connected in line without

any additional hardware. The filter provides a well-

behaved low pass filter response with a wideband VSWR

less than 1.25:1 or 20dB return loss to over 1GHz. It is

suitable for use with input frequencies up to 150MHz

before insertion loss and phase delay begin to compro-

mise the input signal. For frequencies above this range,

consult factory.

Figure 1. DC1076 Filter Diplexer Schematic

Figure 2. DC1076 Filter Diplexer Board

1.000 Inch