Datasheet

LTC6417
24
6417f
applicaTions inForMaTion
In most cases the termination resistor will be matched to
the source resistance, e.g. R
T
= mR
S
. For the LTC6417 with
a wide-band terminated transformer, a plot of output and
input noise density and NF versus termination resistor is
shown in Figure 13. To get the best noise performance in
the system, use the LTC6417 matched to a transformer with
high impedance ratio. Although the output noise density
will be higher, noise figure will improve because of the
additional gain realized in the transformer. An impedance
ratio greater than 8 is not recommended, as the increased
termination resistance with the LTC6417 input capacitance
will limit signal bandwidth. Consult Table 2 for a quick
estimate of the LTC6417’s output noise density and NF
for different transformer impedance ratios. Measured NF
numbers will be higher as the simple noise model does
not take the insertion loss in the transformer into account.
Table 2. Output Noise Density and NF of the LTC4617 with a
Wide-Band Terminated Transformer, R
S
= 50Ω
TRANSFORMER
IMPEDANCE
RATIO m
TERMINATION
RESISTOR R
T
(Ω)
GAIN
(V/V)
OUTPUT NOISE
DENSITY e
no
(nV/√Hz)
NF
(dB)
1 50 1.0 1.57 11.2
2 100 1.4 1.64 8.9
4 200 2.0 1.80 7.0
8 400 2.8 2.14 5.9
Interfacing the LTC6417 to A/D Converters
The LTC6417 has been specially designed to interface
directly with high speed A/D converters. It is possible
to drive the ADC directly from the LTC6417. In practice,
however, better performance may be obtained by adding
a few external components at the output of the LTC6417.
Figure 5 shows the LTC6417 driving an LTC2209 16-bit
ADC. The differential outputs of the LTC6417 are bandpass
filtered, then drive the differential inputs of the LTC2209.
In many applications, a filter like this is desirable to limit
the wideband noise of the amplifier. This is especially
true in high performance 16-bit designs. The minimum
recommended network between the LTC6417 and the ADC
is simply two 10Ω series resistors, which are used to help
eliminate resonances associated with the stray capacitance
of PCB traces and the stray inductance of the internal bond
wires at the ADC input pins and the driver output pins.
Single-Ended Signals
The LTC6417 has not been designed to convert single-
ended signals to differential signals. A single-ended input
signal can be converted to a differential signal via a balun
connected to the inputs of the LTC6417. Figure 5 shows
the LTC6417 driven by a 1:4 transformer which provides
6dB of voltage gain while also performing a single-ended
to differential conversion.
Power Supply Considerations
For best linearity, the LTC6417 should have a positive
supply of V
+
= 5V. For ESD protection, the LTC6417 has
an internal edge-triggered supply voltage clamp. The
timing mechanism of the clamp enables the LTC6417’s
protection circuit during ESD events. This internal clamp
can also be activated by voltage overshoot and rapid slew
rate on the positive supply V
+
pin. The LTC6417 should not
be hot-plugged into a powered socket because there is a
risk of activating this internal ESD clamp circuit. Bypass
capacitors of 680pF and 0.1µF should be connected to the
V
+
pin, as close as possible to the LTC6417.
Interfacing the LTC6417 with Active Mixers for
Ultrawide IF Bandwidth
The LTC6417 is an excellent interface amplifier for use with
active downconverting mixers like the LTC5567. By using
Figure 13. LTC4617 Output and Input Noise
Density and NF vs Termination Resistance
TERMINATION RESISTANCE (Ω)
e
no
(nV/√Hz)
NF (dB)
6417 F13
50 150 400100 200 300 350250
2.3
2.1
1.1
1.3
0.9
1.7
1.9
1.5
0.7
12
11
6
7
5
9
10
8
4
NF
e
no
e
ni