Texas Instruments THS4503EVM manual Single-Endedto Fully Differential Application

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:

-R11 is installed and the measuring instrument is set to 1-MΩ input impedance

or

-R11 is not installed and the measuring instrument has an input impedance of 50 Ω.

Figure 3−1. Single-Ended In/Single-Ended Out, Utilizing Transformer

J1

Vin−

TP1

Vocm

C13

1 µF

C1 0 R2

374Ω

R1

56.2Ω

C2 0 R3

402Ω

R17

0

R4

392Ω

	+VS
	3	R6 0	R8		T1
				4	3	J4
	U1
1	−	4	340Ω			Vout
2	Vocm		R10	5	R11
			280Ω			Ω
8	+	5	R9		49.9
		R7 0		6	1
	THS4503
			340Ω
	6			ADP4−1WT

−VS

R5

392Ω

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 VOCM pin determines the dc operating point of the circuit. When no voltage is connected to TP1, the VOCM voltage level is 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