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TC7126CPL

3-1/2 Digit Analog-to-Digital Converters

Microchip Technology 

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 100μA.

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 1μA.

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 15Ω 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 500Ω

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.

TC7126/A

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.

200

180

No

160

Maximum

Specified

140

Typical

120

100

No

Maximum

80

Maximum

Specified

Typical

60

40

Typical

20

TC7126A

ICL7126

ICL7136

0

FIGURE 7-3:

Coefficient

Analog Common Temp.

9V

+

26

1

V-

V+

240kΩ

TC7126A

VREF+ 36

VREF

VREF- 35

ANALOG 32

COMMON

SET VREF = 1/2 VREF

10kΩ

FIGURE 7-4:

TC7126A Internal Voltage

Reference Connection

© 2006 Microchip Technology Inc.

DS21458C-page 15

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