LT6600-2.5
U
W U U
APPLICATIO S I FOR ATIO
source, provided it remains at least 1.5V above V– and at
least 1.5V below V+. An internal resistor divider sets the
voltage of Pin 7. While the internal 11k resistors are well
matched, their absolute value can vary by ±20%. This
should be taken into consideration when connecting an
external resistor network to alter the voltage of Pin 7.
amplifier inside the LT6600-2.5, and therefore sets the
common mode output voltage of the filter. Since, in the
example of Figure 3, Pin 2 differs from Pin 7 by 0.5V, an
additional 625µA (312µA per side) of DC current will flow
in the resistors coupling the 1st differential amplifier
output stage to filter output. Thus, a total of 3.125mA is
used to translate the common mode voltages.
Pin 2 can be shorted to Pin 7 for simplicity. If a different
common mode output voltage is required, connect Pin 2
to a voltage source or resistor network. For 3V and 3.3V
supplies the voltage at Pin 2 must be less than or equal to
the mid supply level. For example, voltage (Pin 2) ≤ 1.65V
on a single 3.3V supply. For power supply voltages higher
than 3.3V the voltage at Pin 2 can be set above mid supply.
The voltage on Pin 2 should not exceed 1V below the
voltage on Pin 7. The voltage on Pin 2 should not be more
than 2V above the voltage on Pin 7. Pin 2 is a high
impedance input.
A simple modification to Figure 3 will reduce the DC
common mode currents by 36%. If Pin 7 is shorted to
Pin 2 the common mode output voltage of both op amp
stages will be 2V and the resulting DC current will be 2mA.
Of course, by AC coupling the inputs of Figure 3, the
common mode DC current can be reduced to 625µA.
Noise
The noise performance of the LT6600-2.5 can be evalu-
ated with the circuit of Figure 6.
TheLT6600-2.5wasdesignedtoprocessavarietyofinput
signals including signals centered around the mid-supply
voltage and signals that swing between ground and a
positive voltage in a single supply system (Figure 1). The
range of allowable input common mode voltage (the
average of VIN and VIN in Figure 1) is determined by
the power supply level and gain setting (see “Electrical
Characteristics”).
Given the low noise output of the LT6600-2.5 and the 6dB
attenuation of the transformer coupling network, it will be
necessary to measure the noise floor of the spectrum
analyzer and subtract the instrument noise from the filter
noise measurement.
+
–
Example: With the IC removed and the 25Ω resistors
grounded, Figure 6, measure the total integrated noise
(eS) of the spectrum analyzer from 10kHz to 2.5MHz. With
the IC inserted, the signal source (VIN) disconnected, and
the input resistors grounded, measure the total integrated
noise out of the filter (eO). With the signal source con-
nected, set the frequency to 100kHz and adjust the ampli-
tude until VIN measures 100mVP-P. Measure the output
amplitude, VOUT, and compute the passband gain
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the
LT6600-2.5 not only provides lowpass filtering but also
level shifts the common mode voltage of the input signal,
DC currents will be generated through the DC path be-
tweeninputandoutputterminals.Minimizethesecurrents
to decrease power dissipation and distortion.
2.5V
0.1µF
Consider the application in Figure 3. Pin 7 sets the output
common mode voltage of the 1st differential amplifier
inside the LT6600-2.5 (see the “Block Diagram” section)
at 2.5V. Since the input common mode voltage is near 0V,
there will be approximately a total of 2.5V drop across the
series combination of the internal 1580Ω feedback resis-
tor and the external 402Ω input resistor. The resulting
1.25mA common mode DC current in each input path,
must be absorbed by the sources VIN+ and VIN–. Pin 2 sets
the common mode output voltage of the 2nd differential
SPECTRUM
COILCRAFT
ANALYZER
R
R
3
TTWB-1010
1:1
IN
IN
V
IN
INPUT
1
7
2
8
25Ω
–
4
+
LT6600-2.5
50Ω
–
5
+
25Ω
0.1µF
6
66002 F06
–2.5V
Figure 6
660025i
8
Linear [ Linear ]
