3-54

TELCOM SEMICONDUCTOR, INC.

30

20

10

0

0.1/T

1/T

10/T

INPUT FREQUENCY

T = MEASUREMENT

PERIOD

EOC

AZ

Updated Data

Ready

Updated Data

Ready

INT

IZ

AZ

Conversion

Phase

Data to Serial

Port Transmit

Register

Figure 3. Integrating Converter Normal Mode Rejection

Figure 2. A/D Converter Timing

DETAILED DESCRIPTION

Dual Slope Integrating Converter

The TC530/534 dual slope converter operates by inte-

grating the input signal for a fixed time period, then applying

an opposite polarity reference voltage while timing the

period (counting clocks pulses) for the integrator output to

cross 0V (deintegrating). The resulting count is read as

conversion data.

A simple mathematical expression that describes dual

slope conversion is:

(1) Integrate Voltage = Deintegrate Voltage

(2)

0

0

from which:

(3)

[]

[]

and therefore:

[]

Inspection of equation (4) shows dual slope converter

accuracy is unrelated to integrating resistor and capacitor

values, as long as they are stable throughout the measure-

ment cycle. This measurement technique is inherently

Another inherent benefit is noise immunity. Input noise

spikes are integrated (or averaged to zero) during the

integration period. The integrating converter has a noise

immunity with an attenuation rate of at least –20dB per

decade. Interference signals with frequencies at integral

multiples of the integration period are, for the most part,

completely removed. For this reason, the integration period

of the converter is often established to reject 50/60Hz line

noise. The ability to reject such noise is shown by the plot of

Figure 3.

In addition to the two phases required for dual slope

measurement (Integrate and Deintegrate), the TC530/534

performs two additional adjustments to minimize measure-

ment error due to system offset voltages. The resulting four

internal operations (conversion phases) performed each

measurement cycle are: Auto Zero (AZ), Integrator Output

Zero (IZ), Input Integrate (INT) and Reference Deintegrate

conversion.

5V PRECISION DATA ACQUISITION

SUBSYSTEMS

TC530

TC534