Instrumentation
Replacing RSET with a thermistor creates a predictable, albeit nonlinear, temperature-to-frequency generator (Fig. 5a). Thermistors with resistance values covering a wide temperature range that fit conveniently within a single divider setting of the master oscillator can be selected. By experimenting with a spreadsheet, additional series and parallel resistor values can be calculated to improve output frequency linearity for specific thermistors and temperature ranges. Even over temperature, the LTC1799 in this circuit will contribute less than ±0.5°C frequency error.
Humidity can be one of the most difficult environmental parameters to measure. Jim Williams’ novel approach uses an RPO in an AM heterodyne circuit as an interface for capacitive RH sensors (Fig. 5b). The sensor controls a variable oscillator, which is mixed with a reference frequency provided by the LTC1799 RPO. The demodulated difference frequency at the output is a 0- to 1-kHz signal corresponding to 0% to 100% RH. Using an RPO in this application makes calibration a snap. Plus, noise is minimized because the circuit allows one leg of the sensor to be grounded. The circuit contributes approximately 400 ppm/°C of error and provides a PSRR of less than 1% from 4.5 to 5.5 V.
Filter Circuits
Switched-capacitor filters frequently require a nonstandard, or even tunable, reference frequency. Because the filter response will vary with this frequency, it’s critical that the source is stable over time, temperature, and voltage. RPOs are a natural fit for this application. Figure 6 illustrates a simple, precise, 60-Hz tunable notch filter. The LTC1062 and op amp provide the filtering, while the LTC1799 supplies the reference clock. Common notch frequencies are listed in the table. Not only is this circuit flexible, it also offers very high performance, with sharp slopes and over 45 dB of attenuation at the notch frequency
Tips And Tricks
Most circuits are simple, and are usually up and running in a few minutes. But keep in mind that the datasheets and application notes for RPOs provide a wealth of information for developers who adapt them for use in nonstandard applications–or looking to eke out the last drop of performance.
Choosing A Resistor: Because the period of the output signal has a linear relationship with RSET, it’s important to choose wisely. Errors in initial tolerance or in the temperature coefficient add 1 for 1 to the generated frequency error. Low tempco, precision metal-film resistors between 10 and 200 kÙ work well. This corresponds to a master-oscillator frequency of 0.5 to 10 MHz.
Managing Jitter: Jitter can be a problem in certain applications, especially when the frequency must be varied over a wide range. Each divider of the master oscillator has a jitter versus frequency signature. Because the frequency ranges for the dividers overlap, it’s a good idea to choose the one offering the least jitter over the required frequency span. In general, the divider should be set to obtain the lowest master oscillator frequency, as the IC will draw less power and offer better accuracy.
Watch The Layout: The set pin typically can’t tolerate lots of capacitance, so keep parasitics under 10 pF. This isn’t tough to do with a clean pc-board layout. But it can be a "gotcha" during debug if you have a poor layout, or use a high-capacitance scope probe. For best results, RSET should be placed close to the SET pin. Symptoms are inaccuracies due to supply bounce at high frequencies, and increased jitter.
Remote Transducers: In applications such as the thermistor interface discussed earlier, the transducer may be mounted at the end of a long cable, remote from the RPO. In this case, the effective capacitance can be minimized by "bootstrapping" the cable shield to the RSET voltage (Fig. 7).
Special thanks go to Doug LaPorte, Philip Karantzalis, Nello Sevastopoulos, and Jim Williams for their wisdom in preparing this article.