TC7126 datasheet(15/26 Pages) MICROCHIP

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DS21458C-page 15

TC7126/A

7.3

Analog Common (Pin 32)

The analog common pin is set at a voltage potential

approximately 3V below V+. The potential is between

2.7V and 3.35V below V+. Analog common is tied inter-

nally to an N-channel FET capable of sinking 100

μA.

This FET will hold the common line at 3V should an

external load attempt to pull the common line toward

V+. Analog common source current is limited to 1

μA.

Therefore, analog common is easily pulled to a more

negative voltage (i.e., below V+ – 3V).

The TC7126A connects the internal VIN+ and VIN-

inputs to analog common during the auto-zero phase.

During the reference integrate phase, VIN- is con-

nected to analog common. If VIN- is not externally con-

nected to analog common, a Common mode voltage

exists, but is rejected by the converter’s 86dB Common

mode rejection ratio. In battery operation, analog com-

mon and VIN- are usually connected, removing Com-

mon mode voltage concerns. In systems where VIN- is

connected to 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 TC7126A 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 TC7126A’s 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 and a 15

Ω output

impedance.

With sufficiently high total supply voltage (V+ – V- > 7V),

analog common is a very stable potential with excellent

temperature stability (typically 35ppm/°C). This poten-

tial can be used to generate the TC7126A’s reference

voltage. An external voltage reference will be unneces-

sary in most cases because of the 35ppm/°C tempera-

ture coefficient. See Section 7.5 “TC7126A Internal

Voltage

Reference”,

TC7126A

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

resistor. The TEST pin load should be no more than

1mA. See Section 5.0 “DIGITAL SECTION”, Digital

Section for additional 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

TC7126A Internal Voltage

Reference

The TC7126A’s analog common voltage temperature

stability has been significantly improved (Figure 7-3).

The “A” version of the industry standard TC7126 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. Figure 7-4

shows analog common supplying the necessary

voltage reference for the TC7126A.

FIGURE 7-3:

Analog Common Temp.

Coefficient

FIGURE 7-4:

TC7126A Internal Voltage

Reference Connection

Typical

Maximum

Typical

200

180

160

140

120

100

TC7126A

ICL7136

Maximum

Specified

Maximum

Specified

ICL7126

V-

ANALOG

COMMON

TC7126A

VREF+

240k

10k

VREF-

VREF

SET VREF = 1/2 VREF

V+

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