Datasheet

LT6604-15
10
660415fb
APPLICATIONS INFORMATION
In Figure 3 the LT6604-15 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
the passband fl atness near 15MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6604-15 and a current output DAC. The gain, or “tran-
simpedance,” is defi ned as A = V
OUT
/I
IN
. To compute the
transimpedance, use the following equation:
A =
536 •R1
(R1+R2)
(Ω)
By setting R1 + R2 = 536, 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 = 487. 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
MID
•
R1
R1+R2+ 536
+I
IN
•
R1• R2
R1+R2
= 77mV+I
IN
• 45.3Ω
I
IN
is I
IN
+
or I
IN
–
. The transimpedance in this example is
49.8.
Evaluating the LT6604-15
The low impedance levels and high frequency operation
of the LT6604-15 require some attention to the matching
networks between the LT6604-15 and other devices. The
p r e v i o u s e x a m p l e s a ss u m e a n i d e a l ( 0  ) s o u r c e i m p e d a n c e
and a large (1k) load resistance. Among practical examples
–
+
133
133
0.1µF
0.01µF
5V
–
+
V
IN
–
V
IN
+
25
27
4
34
6
2
29
7
660415 F03
V
OUT
+
V
OUT
–
62pF
62pF
+
–
2V
V
t
3
2
1
0
V
IN
+
V
IN
–
V
t
3
2
1
0
V
OUT
+
V
OUT
–
LT6604-15
500mV
P-P
(DIFF)
1/2
–
+
0.1µF
0.01µF
3.3V
–
+
LT6604-15
25
27
V
OUT
+
I
IN
+
I
IN
–
V
OUT
–
4
1/2
34
6
2
29
7
660415 F04
CURRENT
OUTPUT
DAC
R1
R1
R2
R2
=
V
OUT
+
– V
OUT
–
I
IN
+
– I
IN
–
536 • R1
R1 + R2
Figure 3
Figure 4
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6604-15
25
27
4
34
6
2
29
7
660415 F05
402
402
NETWORK
ANALYZER
INPUT
50
COILCRAFT
TTWB-16A
4:1
NETWORK
ANALYZER
SOURCE
COILCRAFT
TTWB-1010
1:1
50
52.3
523
523
1/2
Figure 5