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LTM9002

9002f

APPLICATIONS INFORMATION

Figure 6. Input Termination for Differential

50Ω Input Impedance Using Shunt Resistor

Figure 7. Calculate Differential Gain

The ampliﬁer is unconditionally stable, i.e. differential

stability factor Kf > 1 and stability measure B1 > 0. How-

ever, the overall differential gain is affected by the source

impedance in Figure 7:

AV = | VOUT/VIN | = (500/(RS + ZIN/2)

The noise performance of the ampliﬁer also depends

upon the source impedance and termination. For example,

an input 1:4 transformer in Figure 5 improves the input

noise ﬁgure by adding 6dB 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 input Smith Chart, based on which users can choose

the optimal source impedance for a given gain and noise

requirement.

SENSE Pin Operation

The internal voltage reference can be conﬁgured for two

pin-selectable input ranges of 0.1V (±50mV differential)

or 0.5V (±25mV differential) for LTM9002-AA. Tying the

SENSE pin to VDD selects the higher range; tying the SENSE

pin to 1.5V selects the lower range. For other versions of

LTM9002, the input span is either 2VP-P divided by the

gain or 1VP-P divided by the gain.

An external reference can be used by applying its output

directly or through a resistive divider to SENSE. It is not

recommended to drive the SENSE pin with a logic device.

The SENSE pin should be tied to the appropriate level as

close to the converter as possible. The SENSE pin is inter-

nally bypassed to ground with a 1μF ceramic capacitor.

Input Range

The input range can be set based on the application. The

0.1V input range (LTM9002-AA) will provide the best SNR

performance while maintaining excellent SFDR. The lower

input range will have slightly better SFDR performance, but

the SNR will degrade by 5dB. See the Typical Performance

Characteristics section.

Adjusting the Full-Scale Input Range

To trim the full-scale range of one channel to match that

of the other channel, ﬁrst set the desired range for both

channels by applying an external reference to SENSEA

and SENSEB as shown in Figure 8. Set the DAC codes to

approximately match the external reference voltage. Ap-

ply a full-scale voltage to the input of each channel. Read

the output of both channels and adjust the setting for the

DAC of one channel until the desired channel matching

has been achieved.

The adjustment range and step size depends on the resistor

values chosen for or the source resistance of the external

reference circuit. The external reference is connected to

the SENSE pin which has 10k (±1%) series impedance

with the internal DAC voltage. For the circuit shown in Fig-

ure 8, the step size is 76μV and the code representing 1V

is 0xAAB (0.666748 decimal). In this example, the SENSE

voltage trim range is from approximately 0.79V to 1.1V

including offset and gain errors. Therefore, the effective

input span can be trimmed from ±39.6mV to 55.2mV with

a step size of 3.8μV. However, it is not recommended to

9002 F06

ZIN/2

0.1μF

500Ω

LTM9002

ZIN/2

50Ω

VIN

500Ω

–

0.1μF

IN+

IN–

9002 F07

ZIN/2

500Ω

LTM9002

ZIN/2

RS/2

VIN

500Ω

IN+

IN–

–

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