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LTM9002

9002f

Figure 4. Input Termination for Differential 50Ω

Input Impedance Using Shunt Resistor

Figure 5. Input Termination for Differential 50Ω

Input Impedance Using a Balun

APPLICATIONS INFORMATION

INPUT SPAN

The LTM9002 is conﬁgured with a given input span and

input impedance. With the ampliﬁer gain and the ADC

input network described above for LTM9002-AA, the

full-scale input range of the driver circuit is 0.1VP-P. The

recommended ADC input span is achieved by tying the

SENSE pin to VDD. However, the ADC input span can be

changed if required for the application. The resulting input

span at the IN+/IN– pins is the ADC input span divided by

the gain.

The LTM9002 is intended to be driven through the IN+ and

IN– pins. The DNC pins are used for test purposes and are

not intended to be used in the application. These are test

points within the ADC input ﬁlter network. However, care

should be taken with these pins as they connect directly

to the internal signal path. They should be soldered to an

unconnected pad and should be well isolated.

Input Impedance and Matching

The input impedance of the ampliﬁer is 50Ω, 200Ω or

400Ω depending on the gain of the ampliﬁer. In some

applications the differential inputs may need to be ter-

minated to a lower value impedance, e.g. 50Ω, in order

to provide an impedance match for the source. Several

choices are available.

One approach is to use a differential shunt resistor

(Figure 4). Another approach is to employ a wide band

transformer and shunt resistor (Figure 5). Both methods

provide a wide band match. The termination resistor or

the transformer must be placed close to the input pins in

order to minimize the reﬂection due to input mismatch.

Alternatively, one could apply a narrowband impedance

match at the inputs for frequency selection and/or noise

reduction.

Referring to Figure 6, ampliﬁer inputs can be easily con-

ﬁgured for single-ended input without a balun. The signal

is fed to one of the inputs through a matching network

while the other input is connected to the same matching

network and a source resistor. Because the return ratios

of the two feedback paths are equal, the two outputs have

the same gain and thus symmetrical swing. In general,

the single-ended input impedance and termination resis-

tor RT are determined by the combination of RS, RG and

RF , see Table 5.

Table 4. Differential Ampliﬁer Input Termination Values

GAIN (dB)

ZIN/2

RT FIGURE 4

RT FIGURE 5

200Ω

57Ω

400Ω

100Ω

66.5Ω

None

100Ω

66.5Ω

None

25Ω

None

9002 F04

ZIN/2

500Ω

LTM9002

ZIN/2

25Ω

VIN

500Ω

IN+

IN–

–

9002 F05

ZIN/2

500Ω

LTM9002

ZIN/2

25Ω

VIN

500Ω

–

1:4

•

IN+

IN–

Table 5. Single-Ended Ampliﬁer Input Termination Values

GAIN (dB)

ZIN/2

RT FIGURE 6

200Ω

59Ω

100Ω

68.5Ω

100Ω

66.5Ω

25Ω

150Ω

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