LT6600-20 Linear Technology, LT6600-20 Datasheet - Page 8

• Manufacturer Part Number: LT6600-20
• Description: Very Low Noise Differential Amplifier and 20MHz Lowpass Filter
• Manufacturer: Linear Technology

LT6600-20
In Figure 3 the LT6600-20 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
the passband flatness near 20MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6600-20 and a current output DAC. The gain, or “trans-
impedance,” is defined as A = V
transimpedance, use the following equation:
By setting R1 + R2 = 402 , the gain equation reduces to
A = R1( ).
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 = 49.9 and R2 = 348 . The
voltage at Pin 7 is 1.65V. The voltage at the DAC pins is
given by:
I
50.4 .
Evaluating the LT6600-20
The low impedance levels and high frequency operation of
the LT6600-20 require some attention to the matching
networks between the LT6600-20 and other devices. The
previous examples assume an ideal (0 ) source imped-
ance and a large (1k ) load resistance. Among practical
8
APPLICATIO S I FOR ATIO
IN
A
V
is I
DAC
IN
CURRENT
402
+
OUTPUT
R
DAC
1
or I
26
V
PIN
•
R
mV I
IN
R
I
I
IN
IN
2
7
–
R1
R1
1
–
+
. The transimpedance in this example is
•
R
0.01 F
U
IN
1
•
R
48 3
R
Figure 4
2 402
R2
R2
1
U
.
1
7
2
8
+
–
LT6600-20
OUT
3.3V
6
3
I
+
–
IN
W
66002 F04
/I
0.1 F
4
IN
5
•
. To compute the
R
R R
1
1
•
V
V
R
OUT
OUT
2
2
U
+
–
examples where impedance must be considered is the
evaluation of the LT6600-20 with a network analyzer.
Figure 5 is a laboratory setup that can be used to charac-
terize the LT6600-20 using single-ended instruments with
50 source impedance and 50 input impedance. For a
unity gain configuration the LT6600-20 requires a 402
source resistance yet the network analyzer output is
calibrated for a 50 load resistance. The 1:1 transformer,
53.6
above. The transformer converts the single-ended source
into a differential stimulus. Similarly, the output of the
LT6600-20 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 LT6600-20
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 reflections in
the cabling between the transformer and analyzer input.
Differential and Common Mode Voltage Ranges
The differential amplifiers inside the LT6600-20 contain
circuitry to limit the maximum peak-to-peak differential
voltage through the filter. This limiting function prevents
excessive power dissipation in the internal circuitry
and provides output short-circuit protection. The limiting
function begins to take effect at output signal levels above
2V
illustrated in Figure 6; the LT6600-20 was configured with
unity passband gain and the input of the filter was driven
with a 1MHz signal. Because this voltage limiting takes
place well before the output stage of the filter reaches the
ANALYZER
NETWORK
SOURCE
P-P
50
and it becomes noticeable above 3.5V
and 388
53.6
TTWB-1010
COILCRAFT
1:1
resistors satisfy the two constraints
388
388
Figure 5
1
7
2
8
–
LT6600-20
+
– 2.5V
2.5V
3
6
+
–
0.1 F
0.1 F
4
5
www.DataSheet4U.com
402
402
COILCRAFT
TTWB-16A
4:1
P-P
. This is
ANALYZER
NETWORK
INPUT
66002 F05
66002f
50