LT6604-2.5 Linear Technology, LT6604-2.5 Datasheet - Page 10

• Manufacturer Part Number: LT6604-2.5
• Description: Dual Very Low Noise Differential Amplifier and 2.5MHz Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT6604-2.5
Use Figure 4 to determine the interface between the
LT6604-2.5 and a current output DAC. The gain, or “tran-
simpedance,” is defi ned as A = V
transimpedance, use the following equation:
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 V
Consider Figure 4 with R1 = 49.9Ω and R2 = 1540Ω. The
voltage at V
DAC pins is given by:
Evaluating the LT6604-2.5
The low impedance levels and high frequency operation
of the LT6604-2.5 require some attention to the imped-
ance matching networks between the LT6604-2.5 and
other devices. The previous examples assume an ideal
(0Ω) source impedance and a large (1k) load resistance.
Among practical examples where impedance must be
considered is the evaluation of the LT6604-2.5 with a
network analyzer.
Figure 5 is a laboratory setup that can be used to char-
acterize the LT6604-2.5 using single-ended instruments
with 50Ω source impedance and 50Ω input impedance.
For a 12dB gain confi guration the LT6604-2.5 requires a
402Ω source resistance yet the network analyzer output is
calibrated for a 50Ω load resistance. The 1:1 transformer,
10
CURRENT
A
V
660425 F04
OUTPUT
DAC
DAC
=
1580
(
R
=
=
1
I
I
IN
IN
V
26
R1
R1
+
–
+
MID
MID
R
•
mV
R
2
0.01μF
, for V
1
)
•
( )
+
R
Ω
1
I I
IN
+
R2
S
R2
•
R
= 3.3V, is 1.65V. The voltage at the
48 3Ω
2 1580
R
34
Figure 4
1
4
6
2
+
MID
.
1/2
LT6604-2.5
–
+
25
3.3V
7
and the DAC output current.
+
–
0.1μF
27
29
OUT
+
I
IN
V
/I
OUT
I
IN
IN
•
+
+
R
. To compute the
R R
V
V
– V
– I
OUT
OUT
1
1
IN
OUT
+
•
–
+
–
R
–
2
2
=
1580 • R1
R1 + R2
53.6Ω and 392Ω resistors satisfy the two constraints
above. The transformer converts the single-ended source
into a differential stimulus. Similarly, the output of the
LT6604-2.5 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 LT6604-2.5
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 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 LT6604-2.5 can process
large differential signal levels. On a 3V supply, the output
signal can be 5.1V
signals as large as 8.8V
dissipation in the internal circuitry, the user must limit
differential signal levels to 9V
The two amplifi ers inside the LT6604-2.5 channel have
independent control of their output common mode voltage
(see the “Block Diagram” section). The following guidelines
will optimize the performance of the fi lter.
V
an AC ground with a 0.01μF capacitor or instability may
be observed. V
source, provided it remains at least 1.5V above V
least 1.5V below V
voltage of V
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 V
ANALYZER
NETWORK
MID
SOURCE
50Ω
can be allowed to fl oat, but it must be bypassed to
53.6Ω
MID
TTWB-1010
COILCRAFT
MID
. While the internal 11k resistors are well
1:1
P-P
+
can be driven from a low impedance
. An internal resistor divider sets the
392Ω
392Ω
. Similarly, a 5V supply can support
34
P-P
4
6
2
Figure 5
1/2
LT6604-2.5
+
–
25
. To prevent excessive power
–2.5V
7
2.5V
P-P
+
–
0.1μF
0.1μF
27
29
.
www.DataSheet4U.com
402Ω
402Ω
COILCRAFT
TTWB-16A
4:1
ANALYZER
NETWORK
–
MID
INPUT
and at
660425f
660425 F05
.
50Ω