TC7136ACPI datasheet(14/22 Pages) MICROCHIP

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TC7136/TC7136A

DS21461B-page 14

The TC7136/A connects the internal VIN+ and VIN-

inputs to analog common during the auto-zero phase.

During the reference integrate phase, VIN- is connected

to analog common. If VIN- is not externally connected to

analog common, a Common mode voltage exists, but

is rejected by the converter's 86dB Common mode

rejection ratio. In battery operation, analog common

and VIN- are usually connected, removing Common

mode voltage concerns. In systems where VIN- is con-

nected to the power supply ground or to a given

voltage, analog common should be connected to VIN-.

The analog common pin serves to set the analog sec-

tion reference, or common point. The TC7136A is spe-

cifically designed to operate from a battery, or in any

measurement system where input signals are not refer-

enced (float), with respect to the TC7136A power

source. The analog common potential of V+ – 3V gives

a 7V end of battery life voltage. The common potential

has a 0.001%/% voltage coefficient.

With

sufficiently

high

total

supply

voltage

(V+ – V- > 7V), analog common is a very stable poten-

tial with excellent temperature stability (typically

35ppm/°C for TC7136A. This potential can be used to

generate the TC7136A's reference voltage. An external

voltage reference will be unnecessary in most cases,

because of the 35ppm/°C temperature coefficient. See

Section 7.5, TC7136A Internal Voltage Reference

discussion.

7.4

TEST (Pin 37)

The TEST pin potential is 5V less than V+. TEST may

be used as the negative power supply connection for

external CMOS logic. The TEST pin is tied to the inter-

nally generated negative logic supply through a 500

Ω

resistor. The TEST pin load should not be more than

1mA. See Section 8.0, Typical Applications for addi-

tional information on using TEST as a negative digital

logic supply.

If TEST is pulled high (to V+), all segments plus the

minus sign will be activated. DO NOT OPERATE IN

THIS MODE FOR MORE THAN SEVERAL MINUTES.

With TEST = V+, the LCD segments are impressed with

a DC voltage which will destroy the LCD.

7.5

TC7136A Internal Voltage

Reference

The TC7136 analog common voltage temperature sta-

bility has been significantly improved (Figure 7-3). The

"A" version of the industry standard TC7136 device

allows users to upgrade old systems and design new

systems without external voltage references. External

R and C values do not need to be changed; however,

noise performance will be improved by increasing CAZ

(see Section 6.1, Auto-Zero Capacitor). Figure 7-4

shows analog common supplying the necessary

voltage reference for the TC7136/A.

FIGURE 7-3:

ANALOG COMMON

TEMPERATURE

COEFFICIENT

FIGURE 7-4:

TC7136A INTERNAL

VOLTAGE REFERENCE

CONNECTION

Typical

Maximum

Typical

No Maximum

Specified

200

180

160

140

120

100

TC7136

TC7136A

ICL7136

V-

ANALOG

COMMON

TC7136

TC7136A

VREF+

240k

Ω

10k

Ω

VREF-

VREF

Set VREF = 1/2 VREF

V+

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