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PGA460-Q1: How does time decouple work on the EVM?

Martin Heine
Prodigy 60 points
Part Number: PGA460-Q1


I was wondering what the time decouple does and how it works. 

In the datasheet on page 49, the Transducer Time Decoupling Figure 39. shows a N-MOS Transistor between the lower end of the transformer and GND.

In the Schematics of the EVM however, The N-MOS Transistor is connected to C15, while the lower end of the transformer is connected to GND voa R39. In the EVM User's Guide, it says:

"To use the temperate or time decouple features, desolder the R40 resistor, which shorts the coupling capacitor to GND by default."

This enables the temperature decouple feature, (it decouples the capacitor) but how can use the time decouple feature to decouple the transformer (hardware) ?

  • 0
    TI__Mastermind 27950 points
    Martin,
    To clarify, the decouple feature is optional, and can remain floating if not used.

    As you noted, the difference in hardware between time and temperature decoupling is:
    • Time decouple is connected to both the transformer and tuning cap
    • Temperature decouple is connected only to the tuning cap, and the transformer is always grounded.

    This is because time decoupling completely disconnects the secondary side of the transformer from the primary by floating the secondary side. The objective is to minimize noise that would otherwise couple through the transformer. This may only be useful for systems exposed to excessive noise near the resonant frequency of the transducer. Otherwise, the DSP should be able to filter out typical out-of-band noise.

    Temp decoupling only applies to the tuning capacitor to rectify the matching equivalent accross temperature. Some transducers resonate and respond at off-center frequencies when exposed to extreme temperatures. Thus, by introducing a tuning cap at a specific temperature, the transformer matching circuit can be corrected.

    The EVM does not use the decouple feature in its default state. To enable time decoupling, the description must be expanded to say,
    "To use the time decouple mode, desolder R40, and rotate R39 so that pin six of the transformer bridges to the floating pad of R40 to share the drain net of Q3." The gap between the R39<-->R40 pads will accommodate the 1206 SMD size of R39.