The voltage-to-frequency conversion technique also lends itself to electrostatic guarding of the input circuits. To permit successful and accurate readings in the presence of noise, a large amount of filtering was usually added at the input, but this limited measuring speed by slowing response. A serious problem has been the effects of superimposed noise on the accuracy of the measurement, since noise yields a slightly different number with each scan. Improvements with time have increased the ability of the digital voltmeter to cope with a variety of measurement problems. The basic simplicity of the ramp technique has resulted in reliable and economical voltmeters with typical accuracies of better than 0.05%. This time interval, which is proportional to the input voltage, is measured by a built-in electronic counter to obtain a digital indication of the input voltage. In a step towards achieving speed and accuracy at reduced cost, Hewlett-Packard in 1959 developed a digital voltmeter which depends on measuring the time required for an internal linear voltage ramp to pass from a reference level to a level equal to the unknown DC input voltage. These instruments, however, were relatively slow-responding and expensive. Early digital voltmeters discussed above, HP 405A and HP 3440A, used a null balance or potentiometric system to convert the unknown voltage into a digital presentation of that voltage.
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