LTC6401-20 Datasheet, PDF (11/16 Pages) Linear Technology – 1.3GHz Low Noise, Low Distortion Differential ADC Driver for 140MHz IF

LTC6401-20
APPLICATIONS INFORMATION
R_{S 0.1μF}
50Ω
–+
V_IN
R_T
66.5Ω
0.1μF
13 +IN
14 +IN
100Ω
IN+
15 –IN
R_{S 0.1μF}
50Ω
R_T
66.5Ω
16 –IN
IN–
100Ω
1000Ω
OUT–
OUT+
1000Ω
LTC6401-20
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
–OUTF 6
12.5Ω
–OUT 5
640120 F03
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
two outputs have the same gain and thus symmetrical
swing. In general, the single-ended input impedance and
termination resistor R_T are determined by the combination
of R_S, R_G and R_F. For example, when R_S is 50Ω, it is found
that the single-ended input impedance is 200Ω and R_T is
66.5Ω in order to match to a 50Ω source impedance.
The LTC6401-20 is unconditionally stable. However, the
overall differential gain is affected by both source imped-
ance and load impedance as shown in Figure 4:
A_V =
V_OUT
V_IN
= ²⁰⁰⁰ • ^RL
R_S + 200 25 + R_L
The noise performance of the LTC6401-20 also depends
upon the source impedance and termination. For example,
an input 1:4 balun transformer in Figure 2 improves SNR
by adding 6dB of gain at the inputs. A trade-off between
gain and noise is obvious when constant noise figure
circle and constant gain circle are plotted within the same
1/2 R_S
13 +IN
100Ω
1000Ω
–+ ^VIN
14 +IN
15 –IN
IN+
OUT–
IN–
OUT+
LTC6401-20
12.5Ω
1/2 R_L
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
V_OUT
–OUTF 6
1/2 R_S
16 –IN
100Ω
1000Ω
12.5Ω
1/2 R_L
–OUT 5
640120 F04
Figure 4. Calculate Differential Gain
input Smith Chart, based on which users can choose the
optimal source impedance for a given gain and noise
requirement.
Output Match and Filter
The LTC6401-20 can drive an ADC directly without
external output impedance matching. Alternatively, the
differential output impedance of 25Ω can be matched to
higher value impedance, e.g. 50Ω, by series resistors or
an LC network.
The internal low pass filter outputs at +OUTF/–OUTF
have a –3dB bandwidth of 590MHz. External capacitor
can reduce the low pass filter bandwidth as shown in
Figure 5. A bandpass filter is easily implemented with
only a few components as shown in Figure 6. Three
39pF capacitors and a 16nH inductor create a bandpass
filter with 165MHz center frequency, –3dB frequencies at
138MHz and 200MHz.
100Ω
13 +IN
1000Ω
14 +IN
IN+
OUT–
15 –IN
IN–
OUT+
100Ω
16 –IN
1000Ω
LTC6401-20
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
–OUTF 6
12.5Ω
–OUT 5
640120 F05
8.2pF
12pF
FILTERED OUTPUT
(87.5MHz)
8.2pF
Figure 5. LTC6401-20 Internal Filter Topology Modified for Low
Filter Bandwidth (Three External Capacitors)
100Ω
13 +IN
1000Ω
14 +IN
IN+
OUT–
15 –IN
IN–
OUT+
100Ω
16 –IN
1000Ω
LTC6401-20
12.5Ω
39pF
10Ω
+OUT 8
50Ω
+OUTF 7
50Ω 1.7pF
16nH
–OUTF 6
12.5Ω
10Ω
–OUT 5
39pF
640120 F06
4.99Ω
39pF
4.99Ω
LTC2208
Figure 6. LTC6401-20 Application Circuit for Bandpass
Filtering (Three External Capacitors, One External Inductor)
640120f
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