LTC6400-8_15 Datasheet, PDF (10/16 Pages) Linear Technology – 2.2GHz Low Noise, Low Distortion Differential ADC Driver for DC-300MHz

LTC6400-8
APPLICATIONS INFORMATION
25Ω
200Ω
13 +IN
500Ω
–+ ^VIN
14 +IN
57.6Ω
15 –IN
IN+
OUT–
IN–
OUT+
LTC6400-8
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 2.7pF
–OUTF 6
25Ω
200Ω
16 –IN
500Ω
12.5Ω
–OUT 5
64008 F01
Figure 1. Input Termination for Differential 50Ω Input Impedance
Using Shunt Resistor
R_{S 0.1μF}
50Ω
–+
V_IN
R_T
59.0Ω
0.1μF
13 +IN
14 +IN
200Ω
IN+
15 –IN
0.1μF
27.4Ω
16 –IN
IN–
200Ω
500Ω
OUT–
OUT+
500Ω
LTC6400-8
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 2.7pF
–OUTF 6
12.5Ω
–OUT 5
64008 F03
Figure 3. Input Termination for Single-Ended 50Ω Input
Impedance
25Ω
200Ω
13 +IN
1:4
IN+
–+ ^VIN
•
14 +IN
402Ω
15 –IN
IN–
500Ω
OUT–
OUT+
LTC6400-8
12.5Ω
+OUT 8
50Ω
+OUTF 7
50Ω 2.7pF
–OUTF 6
25Ω
16 –IN
MINI CIRCUITS
TCM4-19
200Ω
500Ω
12.5Ω
–OUT 5
64008 F02
Figure 2. Input Termination for Differential 50Ω Input Impedance
Using a Balun
order to minimize the reflection due to input mismatch.
Alternatively, one could apply a narrowband impedance
match at the inputs of the LTC6400-8 for frequency selec-
tion and/or noise reduction.
Referring to Figure 3, LTC6400-8 can be easily configured
for single-ended input and differential output without a
balun. The signal is fed to one of the inputs through a
matching network while the other input is connected to
the same matching network and a source resistor. Because
the return ratios of the two feedback paths are equal, the
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 322Ω and R_T is
59Ω in order to match to a 50Ω source impedance.
The LTC6400-8 is unconditionally stable, i.e. differential
stability factor Kf>1 and stability measure B1>0. However,
the overall differential gain is affected by both source
impedance and load impedance as shown in Figure 4:
A_V =
V_OUT
V_IN
= ¹⁰⁰⁰ • ^RL
R_S + 400 25 + R_L
The noise performance of the LTC6400-8 also depends upon
the source impedance and termination. For example, an
input 1:4 transformer in Figure 2 improves SNR by adding
6dB 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 input Smith Chart, based
on which users can choose the optimal source impedance
for a given gain and noise requirement.
1/2 R_S
13 +IN
200Ω
500Ω
–+ ^VIN
14 +IN
15 –IN
IN+
OUT–
IN–
OUT+
LTC6400-8
12.5Ω
1/2 R_L
+OUT 8
50Ω
+OUTF 7
50Ω 2.7pF
V_OUT
–OUTF 6
1/2 R_S
16 –IN
200Ω
500Ω
12.5Ω
1/2 R_L
–OUT 5
64008 F04
Figure 4. Calculate Differential Gain
64008f
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