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TELCOM SEMICONDUCTOR, INC.

TC7135

4-1/2 DIGIT

ANALOG-TO-DIGITAL CONVERTER

Table 1. Internal Analog Gate Status

Conversion

Reference

Cycle Phase

+

–

Schematic

System Zero

Closed

Closed

Closed

3B

Input Signal

Closed

3C

Integration

Reference Voltage

Closed*

Closed

3D

Integration

Integrator

Closed

Closed

3E

Output Zero

Internal Analog Gate Status

System Zero Phase

During this phase, errors due to buffer, integrator, and

comparator offset voltages are compensated for by charg-

voltage. With zero input voltage, the integrator output re-

mains at zero.

The external input signal is disconnected from the

internal input points connect to analog common. The refer-

ence capacitor charges to the reference voltage potential

sate for buffer amplifier, integrator, and comparator offset

voltages. (See Figure 3B.)

Analog Input Signal Integration Phase

The TC7135 integrates the differential voltage between

the +INPUT and –INPUT. The differential voltage must be

within the device's common-mode range; –1V from either

supply rail, typically.

The input signal polarity is determined at the end of this

phase. (See Figure 3C.)

Reference Voltage Integration Phase

The previously-charged reference capacitor is con-

nected with the proper polarity to ramp the integrator

output back to zero. (See Figure 3D.) The digital reading

displayed is:

Reading = 10,000

.

Integrator Output Zero Phase

This phase guarantees the integrator output is at 0V

when the system zero phase is entered and that the true

system offset voltages are compensated for. This phase

normally lasts 100 to 200 clock cycles. If an overrange

condition exists, the phase is extended to 6200 clock cycles.

(See Figure 3E.)

Differential Input

Analog Section Functional Description

Differential Inputs

The TC7135 operates with differential voltages (+IN-

PUT, pin 10 and –INPUT, pin 9) within the input amplifier

common-mode range which extends from 1V below the

positive supply to 1V above the negative supply. Within this

common-mode voltage range, an 86 dB common-mode

rejection ratio is typical.

The integrator output also follows the common-mode

voltage and must not be allowed to saturate. A worst-case

condition exists, for example, when a large positive com-

mon-mode voltage with a near full-scale negative differential

input voltage is applied. The negative input signal drives the

integrator positive when most of its swing has been used up

by the positive common-mode voltage. For these critical

applications, the integrator swing can be reduced to less

than the recommended 4V full-scale swing, with some loss

of accuracy. The integrator output can swing within 0.3V of

either supply without loss of linearity.

Analog Common

ANALOG COMMON (pin 3) is used as the –INPUT

return during the auto-zero and deintegrate phases. If

– INPUT is different from analog common, a common-mode

voltage exists in the system. This signal is rejected by the

excellent CMRR of the converter. In most applications,

– INPUT will be set at a fixed known voltage (power supply

common, for instance). In this application, analog common

should be tied to the same point, thus removing the common-

mode voltage from the converter. The reference voltage is

referenced to analog common.

Reference Voltage

The reference voltage input (REF IN, pin 2) must be a

positive voltage with respect to analog common. Two refer-

ence voltage circuits are shown in Figure 5.

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