TC820CLW データシートの表示（PDF） - Microchip Technology

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TC820CLW

3-3/4 digit A/D converter with frequency counter and logic probe

Microchip Technology 

When the logic probe function is selected while FREQ/

VOLTS is low (A/D mode), the ADC will remain in the

Auto-Zero mode. The LCD will read "OL" and all

decimal points will be off (Figure 3-6).

FIGURE 3-6:

High *

Low **

* "High" Annuciator will be on when DP1/HI = Logic High

** "Low" Annunciator and Buzzer will be on when DP0/LO = Logic High

If the logic probe is active while FREQ/VOLTS is high

(Counter mode), the frequency counter will continue to

operate. The display will read "OL" but the decimal

points will be visible. If the logic probe input is also con-

nected to the RANGE/FREQ input, bringing the LOGIC

input low will immediately display the frequency at the

logic probe input.

3.5 Analog Pin Functional Description

3.5.1

DIFFERENTIAL SIGNAL INPUTS

(VIN+), (VIN-)

The TC820 is designed with true differential inputs, and

accepts input signals within the Input Stage Common

mode voltage (VCM) range. The typical range is

VDD – 1V to VSS + 1.5V. Common mode voltages are

removed from the system when the TC820 operates

from a battery or floating power source (isolated from

measured system) and VSS is connected to analog

common (see Figure 3-7).

In systems where Common mode voltages exist, the

86dB 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, positive VCM exists in conjunction with a full

scale, negative differential signal. The negative signal

drives the integrator output positive along with VCM

(Figure 3-8). For such applications, the integrator out-

put swing can be reduced below the recommended 2V

full scale swing. The integrator output will swing within

0.3V of VDD, or VDD without increased linearity error.

3.5.2

REFERENCE (VDD, VSS)

The TC820 reference, like the analog signal input, has

true differential inputs. In addition, the reference volt-

age can be generated anywhere within the power sup-

ply voltage of the converter. The differential reference

inputs permit ratiometric measurements and simplify

interfacing with sensors, such as load cells and temper-

ature sensors.

TC820

To prevent rollover type errors from being induced by

large Common mode voltages, CREF should be large

compared to stray node capacitance. A 0.1µF capacitor

is typical.

The TC820 offers a significantly improved analog com-

mon temperature coefficient, providing a very stable

voltage suitable for use as a voltage reference. The

temperature coefficient of analog common is typically

35ppm/°C.

3.5.3 ANALOG COMMON

The analog common pin is set at a voltage potential

approximately 3.3V below VDD. This potential is

between 3.15V and 3.45V below VDD. Analog common

is tied internally to an N-channel FET capable of sink-

ing 3mA. This FET will hold the common line at 3.3V

below VDD should an external load attempt to pull the

common line toward VDD. Analog common source cur-

rent is limited to 12µA, and is, therefore, easily pulled to

a more negative voltage (i.e., below VDD – 3.3V).

The TC820 connects the internal VIN+ and VIN- inputs

to analog common during the auto-zero cycle. 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.

This is rejected by the converter's 86dB Common mode

rejection ratio. In battery powered applications, analog

common and VIN- are usually connected, removing

Common mode voltage concerns. In systems where

VIN- is connected 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 TC820 is specifi-

cally designed to operate from a battery, or in any

“measurement" system where input signals are not ref-

erenced (float), with respect to the TC820 power

source. The analog common potential of VDD – 3.3V

gives a 7V end of battery life voltage. The analog com-

mon potential has a voltage coefficient of 0.001%.

With a sufficiently high total supply voltage

(VDD – VSS > 7V), analog common is a very stable

potential with excellent temperature stability (typically

35ppm/°C). This potential can be used to generate the

TC820 reference voltage. An external voltage refer-

ence will be unnecessary in most cases, because of

the 35ppm/°C temperature coefficient. See the applica-

tions section for details.

© 2002 Microchip Technology Inc.

DS21476B-page 13

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