Single-Ended In/Single-Ended Out, Utilizing Transformer; Single-Ended To Fully Differential Application - Texas Instruments THS4500EVM 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 THS4500EVM 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
were 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 m F
R2
C1 0
J1
Vin−
374 W
R1
56.2 W
C2 0 R3
402 W
R17
0

3.2 Single-Ended to Fully Differential Application

Single-supply, single-ended to fully differential conversion is a popular
application for fully differential amps. Until the introduction of fully differential
amps such as the THS4500, it was necessary to include two coupling
capacitors to prevent the input signal from violating the input common mode
range of the op amp. The THS4500 is optimized for ground-referenced inputs
in single supply applications—no capacitors are required.
When operated, the the output voltage is offset to +VS/2 (2.5 V) by the internal
action of V
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
392 W
+VS
3
U1
1 4
−
2
Vocm
8 5
+
THS4500
6
−VS
R5
392 W
.
OCM
T1
R6 0 R8
4 3
340 W
R10
5
280 W
R7 0 R9
6 1
340 W
ADP4 − 1WT
J4
Vout
R11
49.9 W