LT6600-2.5
U
W U U
APPLICATIO S I FOR ATIO
In Figure 3 the LT6600-2.5 is providing 12dB of gain. The
common mode output voltage is set to 2V.
Figure 5 is a laboratory setup that can be used to charac-
terize the LT6600-2.5 using single-ended instruments
with 50Ω source impedance and 50Ω input impedance.
For a 12dB gain configuration the LT6600-2.5 requires a
402Ωsourceresistanceyetthenetworkanalyzeroutputis
calibrated for a 50Ω load resistance. The 1:1 transformer,
53.6Ω and 388Ω resistors satisfy the two constraints
above. The transformer converts the single-ended source
into a differential stimulus. Similarly, the output of the
LT6600-2.5 will have lower distortion with larger load
resistance yet the analyzer input is typically 50Ω. The 4:1
turns (16:1 impedance) transformer and the two 402Ω
resistors of Figure 5, present the output of the LT6600-2.5
with a 1600Ω differential load, or the equivalent of 800Ω
to ground at each output. The impedance seen by the
network analyzer input is still 50Ω, reducing reflections in
the cabling between the transformer and analyzer input.
Use Figure 4 to determine the interface between the
LT6600-2.5 and a current output DAC. The gain, or “trans-
impedance,” is defined as A = VOUT/IIN. To compute the
transimpedance, use the following equation:
1580 •R1
A =
Ω
( )
R1+R2
(
)
By setting R1 + R2 = 1580Ω, the gain equation reduces to
A = R1(Ω).
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on Pin 7 and the DAC output current.
Consider Figure 4 with R1 = 49.9Ω and R2 = 1540Ω. The
voltage at Pin 7 is 1.65V. The voltage at the DAC pins is
given by:
Differential and Common Mode Voltage Ranges
R1
R1•R2
R1+R2
V
DAC = VPIN7
•
+I •
IN
Therail-to-railoutputstageoftheLT6600-2.5canprocess
large differential signal levels. On a 3V supply, the output
signal can be 5.1VP-P. Similarly, a 5V supply can support
signals as large as 8.8VP-P. To prevent excessive power
dissipation in the internal circuitry, the user must limit
R1+R2 +1580
= 26mV +I • 48.3Ω
IN
IIN is IIN+ or IIN–. The transimpedance in this example is
49.6Ω.
differential signal levels to 9VP-P
.
Evaluating the LT6600-2.5
The two amplifiers inside the LT6600-2.5 have indepen-
dent control of their output common mode voltage (see
the “Block Diagram” section). The following guidelines
will optimize the performance of the filter.
The low impedance levels and high frequency operation of
the LT6600-2.5 require some attention to the matching
networks between the LT6600-2.5 and other devices. The
previous examples assume an ideal (0Ω) source imped-
ance and a large (1kΩ) load resistance. Among practical
examples where impedance must be considered is the
evaluation of the LT6600-2.5 with a network analyzer.
Pin 7 can be allowed to float; Pin 7 must be bypassed to an
ACgroundwitha0.01µFcapacitororsomeinstabilitymay
be observed. Pin 7 can be driven from a low impedance
2.5V
0.1µF
CURRENT
OUTPUT
DAC
3.3V
COILCRAFT
TTWB-16A
4:1
COILCRAFT
TTWB-1010
NETWORK
ANALYZER
SOURCE
NETWORK
ANALYZER
INPUT
0.1µF
3
388Ω
1:1
1
7
2
8
402Ω
–
4
–
+
3
R2
R2
I
I
IN
1
7
2
8
50Ω
LT6600-2.5
–
4
+
–
53.6Ω
50Ω
V
V
+
OUT
R1
+
–
0.01µF
LT6600-2.5
5
+
402Ω
388Ω
–
6
IN
OUT
0.1µF
5
+
660025 F05
6
R1
660025 F04
–2.5V
Figure 5
Figure 4
660025i
7
Linear [ Linear ]
