LTM9002
21
9002f
APPLICATIONS INFORMATION
Figure 6. Input Termination for Differential
50Ω Input Impedance Using Shunt Resistor
Figure 7. Calculate Differential Gain
The amplifi er 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 = | V
OUT
/V
IN
| = (500/(R
S
+ Z
IN
/2)
The noise performance of the amplifi er also depends
upon the source impedance and termination. For example,
an input 1:4 transformer in Figure 5 improves the input
noise fi gure by adding 6dB 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 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 confi gured for two
pin-selectable input ranges of 0.1V (±50mV differential)
or 0.5V (±25mV differential) for LTM9002-AA. Tying the
SENSE pin to V
DD
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 2V
P-P
divided by the
gain or 1V
P-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, fi rst 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
Z
IN
/2
0.1μF
0.1μF
500Ω
LTM9002
Z
IN
/2
R
S
50Ω
R
S
50Ω
V
IN
500Ω
+
–
0.1μF
R
T
R
T
IN
+
IN
–
9002 F07
Z
IN
/2
R
T
500Ω
LTM9002
Z
IN
/2
R
S
/2
R
S
/2
V
IN
500Ω
IN
+
IN
–
+
–
