TC9400/9401/9402

DS21483B-page 8

© 2002 Microchip Technology Inc.

3.2

Voltage-to-Time Measurements

The TC9400 output can be measured in the time

domain as well as the frequency domain. Some micro-

computers, for example, have extensive timing capabil-

ity, but limited counter capability. Also, the response

time of a time domain measurement is only the period

between two output pulses, while the frequency mea-

surement must accumulate pulses during the entire

counter time-base period.

Time measurements can be made from either the

TC9400's PULSE FREQ OUT output, or from the

FREQ/2 OUT output. The FREQ/2 OUT output

changes state on the rising edge of PULSE FREQ

OUT, so FREQ/2 OUT is a symmetrical square wave at

one-half the pulse output frequency. Timing measure-

ments can, therefore, be made between successive

PULSE FREQ OUT pulses, or while FREQ/2 OUT is

high (or low).

4.0

PIN FUNCTIONS

4.1

Threshold Detector Input

In the V/F mode, this input is connected to the AMPLI-

FIER OUT output (Pin 12) and triggers a 3

µsec pulse

when the input voltage passes through its threshold. In

the F/V mode, the input frequency is applied to this

input.

The nominal threshold of the detector is half way

The TC9400's charge balancing V/F technique is not

dependent on a precision comparator threshold,

because the threshold only sets the lower limit of the

Op Amp output. The Op Amp's peak-to-peak output

swing, which determines the frequency, is only

4.2

Pulse Freq Out

This output is an open drain N-channel FET, which pro-

vides a pulse waveform whose frequency is propor-

tional to the input voltage. This output requires a pull-

up resistor and interfaces directly with MOS, CMOS,

and TTL logic (see Figure 4-1).

FIGURE 4-1:

OUTPUT WAVEFORMS

3

µsec

Typ.

1/f

Amp Out

0V

Notes:

resistors and glass capacitors). Also, separate output ground (Pin 9) from input ground (Pin 6).