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TELCOM SEMICONDUCTOR, INC.

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3-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTERS

TC7126

TC7126A

In some applications, a scale factor other than unity may

exist between a transducer output voltage and the required

digital reading. Assume, for example, a pressure transducer

Required Full-Scale Voltage*

200 mV

100 mV

2V

1V

input voltage by two, the reference voltage should be set to

200 mV. This permits the transducer input to be used

directly.

The differential reference can also be used where a

This is common in temperature-measuring instrumentation.

A compensating offset voltage can be applied between

DEVICE PIN FUNCTIONAL DESCRIPTION

(Pin Numbers Refer to 40-Pin DIP)

Differential Signal Inputs

The TC7126A is designed with true differential inputs

and accepts input signals within the input stage common-

+1V. Common-mode voltages are removed from the system

when the TC7126A operates from a battery or floating power

In systems where common-mode voltages exist, the

TC7126A's 86 dB common-mode rejection ratio minimizes

error. Common-mode voltages do, however, affect the inte-

grator output level. A worst-case condition exists if a large

differential signal. The negative signal drives the integrator

applications, the integrator output swing can be reduced

below the recommended 2V full-scale swing. The integrator

linearity error.

Differential Reference

The reference voltage can be generated anywhere

To prevent roll-over type errors being induced by large

stray node capacitance.

The TC7126A offers a significantly improved analog

common temperature coefficient. This potential provides a

very stable voltage, suitable for use as a voltage reference.

The temperature coefficient of analog common is typically

35 ppm/

°C for the TC7126A and 80 ppm/°C for the TC7126.

ANALOG COMMON (Pin 32)

The analog common pin is set at a voltage potential

tied internally to an N-channel FET capable of sinking

The input buffer amplifier and integrator are designed

with Class A output stages. The output stage idling current

is 6

µA. The integrator and buffer can supply 1 µA drive

remain in the output stage linear drive region, but not so

large that PC board leakage currents induce errors. For a

1.8 M

Ω.

Oscillator Components

equation:

.

For a 48 kHz clock (3 conversions per second), R = 180 k

Ω.

4 to generate the TC7126A's inter-

nal clock. The backplane drive signal is derived by dividing

To achieve maximum rejection of 60 Hz noise pickup,

the signal integrate period should be a multiple of 60 Hz.

Oscillator frequencies of 240 kHz, 120 kHz, 80 kHz, 60 kHz,

40 kHz, etc. should be selected. For 50 Hz rejection,

oscillator frequencies of 200 kHz, 100 kHz, 66-2/3 kHz, 50

kHz, 40 kHz, etc. would be suitable. Note that 40 kHz (2.5

readings per second) will reject both 50 Hz and 60 Hz.

Reference Voltage Selection

A full-scale reading (2000 counts) requires the input

signal be twice the reference voltage.

Component

Nominal Full-Scale Voltage

Value

200 mV

2V

0.33

µF

0.033

µF

180 k

Ω

1.8 M

Ω

0.047

µF

0.047

µF

NOTE:

0.45

RC