Created on 2009-05-05 23:15:00
How can I use the APx585 or 586 to test a DUT that is designed to drive load impedance lower than 100 kΩ (unbalanced) or 200 kΩ (balanced)—say 600 Ω for old professional audio equipment, or 32 Ω for a headphone output?
The APx585 and 586 only offer fixed input termination impedances of 100 kΩ (unbalanced) and 200 kΩ (balanced). Unlike its 2 and 4-channel cousins, the APx520/525 and 521/526, which offer selectable 300, 600, and 100 kΩ unbalanced (200 kΩ balanced) input terminations, the input impedance of the APx585 and 586 is fixed at 100 kΩ unbalanced (200 kΩ balanced). This is due to the high channel count of these units—there just wasn´t enough room to add the resistors and relays to make all the input termination choices available.
It is easy, however, to achieve a lower input termination impedance by simply adding an external resistor in parallel with the input of each channel being used. When two resistors are combined in parallel, their effective resistance Reff can be determined from the classic equation
where, in this case, Rex is the external resistance and Rint represents the instrument input impedance. Note that when Rint is much greater than Rex, the above equation reduces to Reff ≈ Rex, and the input termination impedance is essentially equal to the value of the external resistance. In the case of the APx585 and 586, where Rint = 100 kΩ or 200 kΩ, the above equation can be ignored for external resistance values less than about 1000 Ω.
For unbalanced input connections to APx585/586, the simplest way to add the external resistor is by inserting a BNC-Tee and adapters as shown in Figure 1. For improved noise shielding, a metal shell tied to the connector shield should enclose the adapter containing the resistor. An alternative would be to modify a commercially made 50 Ω or 75 Ω BNC terminator by replacing its resistor with a resistor of the required value.
Figure 1 Unbalanced terminator adapter
For balanced connections, the most convenient way to add an external resistor in parallel with the input is with an XLR barrel adapter, such as the Switchcraft S3FM. Not only is this convenient, but it will not compromise the crosstalk resistance of the cable connection. You will need one adapter for each channel to be converted. The wiring diagram is shown in Figure 2. To connect the adapters to the APx585/586 audio analyzer, you can use the DB25M to 8-XLR female cables included in the CAB-585 cable kit.
Figure 2 Barrel wiring diagram
Don´t forget to consider current limitations, especially with low load impedances. The current should not exceed the power rating of the external resistor, and in any case, these adapters are typically not appropriate for power dissipation exceeding say 1 W.