lt6600cs8-5-trpbf Linear Technology Corporation, lt6600cs8-5-trpbf Datasheet - Page 10

• Manufacturer Part Number: lt6600cs8-5-trpbf
• Description: Very Low Noise, Differential Amplifier And 10mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

LT6600-10
the passband flatness near 10MHz. The common mode
output voltage is set to 2V.
Use Figure 4 to determine the interface between the
LT6600-10 and a current output DAC. The gain, or “trans-
impedance”, is defined as A = V
transimpedance, use the following equation:
By setting R1 + R2 = 402Ω, the gain equation reduces to
A = R1Ω.
The voltage at the pins of the DAC is determined by R1,
R2, the voltage on V
or I
348Ω. The voltage at V
DAC pins is given by:
I
50.4Ω.
Evaluating the LT6600-10
The low impedance levels and high frequency operation of
the LT6600-10 require some attention to the matching
networks between the LT6600-10 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 LT6600-10 with a network analyzer.
10
APPLICATIO S I FOR ATIO
IN
A
V
is I
IN
DAC
=
–
IN
). Consider Figure 4 with R1 = 49.9Ω and R2 =
CURRENT
402
OUTPUT
R
–
=
=
DAC
1
or I
+
V
103
•
PIN
R
R
IN
2
mV I
1
I
I
7
IN
IN
+
Figure 4. (S8 Pin Numbers)
Ω
.The transimpedance in this example is
–
+
•
R
+
U
R1
R1
1
MID
IN
+
43 6
R
0.01μF
MID
and the DAC output current (I
R
2 402
R2
R2
1
.
U
+
Ω
is 1.65V. The voltage at the
OUT
1
7
2
8
+
/I
I
–
+
LT6600-10
IN
W
IN
3.3V
3
6
Ω. To compute the
R
R R
+
–
6600 F04
0.1μF
1
1
4
5
+
•
R
2
2
V
V
U
OUT
OUT
+
–
IN
+
Figure 5 is a laboratory setup that can be used to charac-
terize the LT6600-10 using single-ended instruments with
50Ω source impedance and 50Ω input impedance. For a
unity gain configuration the LT6600-10 requires a 402Ω
source resistance yet the network analyzer output is
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 the
LT6600-10 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 LT6600-10
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.
Differential and Common Mode Voltage Ranges
The differential amplifiers inside the LT6600-10 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 LTC6600-10 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 supply rails, the input/output behavior of the
IC shown in Figure 6 is relatively independent of the power
supply voltage.
NETWORK
ANALYZER
SOURCE
P-P
50Ω
and it becomes noticeable above 3.5V
53.6Ω
TTWB-1010
COILCRAFT
Figure 5. (S8 Pin Numbers)
1:1
388Ω
388Ω
1
7
2
8
LT6600-10
+
–
– 2.5V
3
6
2.5V
+
–
0.1μF
0.1μF
4
5
402Ω
402Ω
COILCRAFT
TTWB-16A
4:1
P-P
. This is
NETWORK
ANALYZER
INPUT
6600fa
6600 F05
50Ω