TC850 datasheet(11/30 Pages) MICROCHIP

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 2001-2012 Microchip Technology Inc.

DS21479D-page 11

TC850

5.0

PIN DESCRIPTION (ANALOG)

5.1

Differential Inputs (IN+ and IN–)

The analog signal to be measured is applied at the IN+

and IN– inputs. The differential input voltage must be

within the Common mode range of the converter. The

input Common mode range extends from VDD - 1.5V to

VSS +1.5V. Within this Common mode voltage range,

an 80 dB CMRR is typical.

The integrator output also follows the Common mode

voltage. The integrator output must not be allowed to

saturate. A worst-case condition exists, for example,

when a large, positive Common 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 available swing has been

used up by the positive Common mode voltage. For

applications where maximum Common mode range is

critical, integrator swing can be reduced. The integrator

output can swing within 0.4V of either supply without

loss of linearity.

5.2

Differential Reference (VREF)

The TC850 requires two reference voltage sources in

order to generate the “fast-slow” de-integrate phases.

The main voltage reference (VREF1) is applied between

the REF1+ and REF- pins. The secondary reference

(VREF2) is applied between the REF2+ and REF- pins.

The reference voltage inputs are fully differential and

the reference voltage can be generated anywhere

within the power supply voltage of the converter. How-

ever, to minimize rollover error, especially at high con-

version rates, keep the reference Common mode

voltage (i.e., REF-) near or at the analog common

potential. All voltage reference inputs are high-imped-

ance. Average reference input current is typically only

30 pA.

5.3

Analog Common (ANALOG

COMMON)

Analog common is used as the IN- return during the

zero integrator and de-integrate phases of each con-

version. If IN- is at a different potential than analog

common, a Common mode voltage exists in the sys-

tem. This signal is rejected by the 80dB CMRR of the

converter. However, in most applications, IN- will be set

at a fixed, known voltage (power supply common, for

instance). In this case, analog common should be tied

to the same point so that the Common mode voltage is

eliminated.

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