AN-D| Application Note

Versatile Monolithic V/Fs Can Compute as Well as Convert with High Accuracy

Versatile Monolithic V/Fs can Compute as Well as Convert with High Accuracy
The best of the monolithic voltage-to-frequency (V/F) converters have performance that's so good it equals or exceeds that of modular types. Some of these ICs can be designed into quite a variety of circuits because they're notably versatile. Along with versatility and high performance come the advantages that are characteristic of all V/F converters, including good linearity, excellent resolution, wide dynamic range, and an output signal that's easy to transmit as well as couple through an isolator. One of the recently introduced monolithic types, the LM131, has both high performance and a design that's rather flexible. For instance, it can compute and convert at the same time; the computation is a part of the conversion. Among other functions, it can provide the product, ratio and square root of analog inputs. This IC has an internal reference for its conversion circuitry that's also brought out to a pin, so it's available to external circuits associated with the converter. Not surprisingly, it turns out t
hat any deviations of the reference, due to process variations and temperature changes have equal and opposite effects on the scale factors of the converter and the external circuitry. (This presumes, of course, that the scale factor of the external circuitry is a linear function of voltage.)

National Semiconductor Application Note D August 1980

Precision Relaxation Oscillator
Before looking at some applications, quickly take a look at the basic circuit of an LM131 V/F converter (Figure 1). Basically, this IC, like any V/F converter, is a precision relaxation oscillator that generates a frequency linearly propor-

tional to the input voltage. As might be expected, the circuit has a capacitor, CL, with a sawtooth voltage on it. Generally speaking, the circuit is a feedback loop that keeps this capacitor charged to a voltage very slightly higher than the input voltage, VIN. If VIN is high, CL discharges relatively quickly through RL, and the circuit generates a high frequency. If VIN is low, CL discharges slowly, and the converter puts out a low frequency. When CL discharges to a voltage equal to the input, the comparator triggers the one-shot. The one-shot closes the current switch and also turns on the output transistor. With the switch closed, current from the current source recharges CL to a voltage somewhat higher than the input. Charging continues for a period determined by RT and CT. At the end of this period, the one-shot returns to its quiescent state and CL resumes discharging. Resistor RS sets the amount of current put out by the current source. In fact, the current in pin 1, with the switch on, is identical
to the current in pin 2. The latter pin is at a constant voltage (nominally 1.90V), so a given resistor value can set the operating currents. When connected to a high impedance buffer, this pin provides a stable reference for external circuits. The open-collector output at pin 3 permits the output swing to be different from the converter's supply voltage, if the load circuit requires. The supplies don't have to be separate, however, and both the converter and its load can use the same voltage.

AN-D

2002 National Semiconductor Corporation

AN008742

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