PGA460: Problems in ultrasonic robot application

Hi Stanley,

Responding to your questions:

1. The analog front end is time varying, hence time varying gain (TVG). The gain profile is predetermined by the user, and not based on preceding samples for reference (such as a log amp). A common time varying gain profile is to gradually increase the gain from an initial low gain level to a large gain level for long distance measurements. Take a look at the PGA460 ultrasonic sensing: EVM GUI device configuration video demo to see the TVG changes in action.

2. The PGA460 does not include the DSP intelligence to suppress or differentiate various ultrasonic echos in the same frequency band of interest. Echo coding is not supported. For a system that is exposed to a noisy ultrasonic environment, I recommend that the time between each measurement iteration be randomized. This will allow for improved recognition of false positives caused by other ultrasonic sources.

3. Transformer selection should be based on two parameters: transducer drive voltage rating and frequency.

• If the transducer drive voltage rating is 50Vpp or greater, then a transformer driver is necessary to generate maximum sound pressure level for maximum ranging of the transducer. Typical ultrasonic transducer center-tap transformers will have a turns ratio of 1:1:10. The drive voltage can be fine-tuned and controlled by the center tap voltage supplied to the transformer and the PGA460 preset driver current limit value (50mA-500mA). 
• If the frequency of the transducer is 30-80kHz, I recommend using a transformer with a 1~5mH secondary inductance rating. If the transducer frequency is 180-480kHz, I recommend using a transformer with a 100~500nH secondary inductance rating.
• You can find a list of PGA460 compatible transducers and transformers at: http://www.ti.com/lit/zip/slac787
• The matching values (tuning capacitor and/or damping resistor in parallel to transducer) will be impacted by the secondary inductance of the transformer, and the transducer's equivalent circuit. You will need to optimize these values based on your min/max range requirements.

Stanley,

Though IOREG is not intended to be used to drive external loads, it is possible to confiugre IOREG to 5V. Keep in mind that configuring IOREG to 5V will also configure TXD, RXD, and SCLK pins will also be referenced to 5V high logic.

You need to connect an external 100kOhm pull up resistor from the TEST pin to 3.3V or 5V source to enable 5V at IOREG and the USART pins. See section 7.3.12 TEST Pin Functionality of the datasheet ( http://www.ti.com/lit/gpn/pga460-q1 ) for details.