LMH3401 Datasheet, PDF (19/48 Pages) Texas Instruments – LMH3401 7-GHz, Ultra-Wideband, Fixed-Gain, Fully-Differential Amplifier

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8.11 Test Schematics
LMH3401
SBOS695A – AUGUST 2014 – REVISED DECEMBER 2014
Test Equipment
With 50-:
Outputs
50-:, Single-Ended
Input
IN-
+
:
VIN
-
IN+
CM
12.5 :
12.5 :
Device
200 :
200 :
:
+
OUT_AMP
-
:
OUT+ ^:
:
OUT-
Differential
Load = 200 :
+
OUT_LOAD
-
 _{Test Equipment}
With 50-:

Inputs
Test Equipment
Load = 100 :
PD
Figure 49. Test Schematic: Single-Ended Input, Differential Output
Test Equipment
With 50-:
Outputs
50-:, Single-Ended
Input
IN-
+
- ^VIN
IN+
CM
12.5 :
12.5 :
Device
200 :
200 :
:
+
OUT_AMP
-
:
OUT+ ^:
:
OUT-
Differential
Load = 200 :
+
OUT_LOAD
-
PD
Figure 50. Test Schematic: Differential Input, No Input Match
 _{Test Equipment}
With 50-:

Inputs
Test Equipment
Load = 100 :
Test Equipment
With 50-:
Outputs
50-:, Single-Ended
Input
37.5 :
+
VIN
-
37.5 :
CM
12.5 :
12.5 :
Device
200 :
200 :
:
+
OUT_AMP
-
:
OUT+ ^:
:
OUT-
Differential
Load = 200 :
+
OUT_LOAD
-
 _{Test Equipment}
With 50-:

Inputs
Test Equipment
Load = 100 :
PD
Figure 51. Test Schematic: Differential Input, Input Matched to 100-Ω Differential
Note that in Figure 50, even though the amplifier gain is A_V = 200 / 12.5 = 16 V/V (or 24 dB) there is a significant
loss at the input resulting from the low input impedance. With 50-Ω test equipment this loss is 12.5 / (50 + 12.5)
= 0.2 V/V (or –10 dB). The loss created by the low input impedance puts the net gain for this circuit at 14 dB,
only slightly higher than the gain for the fully-terminated configuration of 12 dB. In most applications the external
input termination resistors are worth the cost.
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