LT6600-2.5
9
660025fe
APPLICATIONS INFORMATION
Use Figure 4 to determine the interface between the
LT6600-2.5 and a current output DAC. The gain, or “trans-
impedance,” is defi ned as A = V
OUT
/I
IN
. To compute the
transimpedance, use the following equation:
A =
1580 •R1
R1+ R2
()
Ω
()
By setting R1 + R2 = 1580, the gain equation reduces
to A = R1().
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on V
MID
and the DAC output current.
Consider Figure 4 with R1 = 49.9 and R2 = 1540. The
voltage at V
MID
, for V
S
= 3.3V, is 1.65V. The voltage at the
DAC pins is given by:
V
DAC
= V
PIN7
•
R1
R1+ R2 + 1580
+I
IN
•
R1• R2
R1+ R2
= 26mV +I
IN
• 48.3Ω
I
IN
is I
IN
+
or I
IN
–
. The transimpedance in this example is
49.6.
Evaluating the LT6600-2.5
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 (0) source imped-
ance and a large (1k) load resistance. Among practical
examples where impedance must be considered is the
evaluation of the LT6600-2.5 with a network analyzer.
Figure 5 is a laboratory setup that can be used to charac-
terize the LT6600-2.5 using single-ended instruments
with 50 source impedance and 50 input impedance.
For a 12dB gain confi guration the LT6600-2.5 requires a
402 source resistance yet the network analyzer output is
calibrated for a 50 load resistance. The 1:1 transformer,
53.6 and 388 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 50. The 4:1
turns (16:1 impedance) transformer and the two 402
resistors of Figure 5, present the output of the LT6600-2.5
with a 1600 differential load, or the equivalent of 800
to ground at each output. The impedance seen by the
network analyzer input is still 50, reducing refl ections in
the cabling between the transformer and analyzer input.
Differential and Common Mode Voltage Ranges
The rail-to-rail output stage of the LT6600-2.5 can process
large differential signal levels. On a 3V supply, the output
signal can be 5.1V
P-P
. Similarly, a 5V supply can support
signals as large as 8.8V
P-P
. To prevent excessive power
dissipation in the internal circuitry, the user must limit
differential signal levels to 9V
P-P
.
The two amplifi ers 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 fi lter.
Figure 4. (S8 Pin Numbers) Figure 5. (S8 Pin Numbers)
–
+
0.1µF
3.3V
–
+
LT6600-2.5
3
4
1
0.01µF
CURRENT
OUTPUT
DAC
7
2
8
5
V
OUT
+
V
OUT
–
660025 F04
6
R2
R1
I
IN
–
I
IN
+
R2
R1
=
V
OUT
+
– V
OUT
–
I
IN
+
– I
IN
–
1580 • R1
R1 + R2
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-2.5
3
4
1
7
2
8
5
6
660025 F05
402Ω
402Ω
NETWORK
ANALYZER
INPUT
50Ω
COILCRAFT
TTWB-16A
4:1
NETWORK
ANALYZER
SOURCE
COILCRAFT
TTWB-1010
1:1
50Ω
53.6Ω
388Ω
388Ω