Single-Ended In/Single-Ended Out, Utilizing Transformer; Single-Ended To Fully Differential Application - Texas Instruments THS4503EVM User Manual

Single-Ended In/Single-Ended Out, Utilizing Transformer

3.1 Single-Ended In/Single-Ended Out, Utilizing Transformer

The fully differential amp output can be monitored by a single-ended
instrument at J4. The THS4503EVM utilizes Mini-Circuits CD542 footprint
transformers to make the fully differential to single-ended conversion. An
ADP4−1WT transformer is installed on the board.
R8, R9, and R10 are chosen such that the load on the fully differential amp is
800 Ω when combined with the load impedance transformed by the turn ratio
T1. This load is chosen because it is a common input impedance value for
ADCs, and is the impedance at which many fully differential amp parameters
are measured. The 800-Ω load occurs when one of two conditions is met:
-
or
-
Figure 3−1. Single-Ended In/Single-Ended Out, Utilizing Transformer
C13
1 µF
R2
C1 0
J1
Vin−
374Ω
R1
56.2Ω
C2 0 R3
402Ω
R17
0

3.2 Single-Ended to Fully Differential Application

The schematic of Figure 3−2 shows the proper technique for ac-coupling. The
voltage present on the V
circuit. When no voltage is connected to TP1, the V
determined by a voltage divider internal to the op amp, and is approximately
equal to half of +VS. This dc voltage is present on both outputs, and also
present on both inputs—being connected through R2 and R3.
3-2
R11 is installed and the measuring instrument is set to 1-MΩ input
impedance
R11 is not installed and the measuring instrument has an input impedance
of 50 Ω.
TP1
Vocm
R4
392Ω
+VS
3
U1
1 4
−
2
Vocm
8 5
+
THS4503
6
− VS
R5
392Ω
OCM
T1
R6 0 R8
4
340Ω
R10
5
280Ω
R7 0 R9
6
340Ω
ADP4 − 1WT
pin determines the dc operating point of the
OCM
3 J4
Vout
R11
49.9Ω
1
voltage level is