TC7116 datasheet(12/24 Pages) MICROCHIP

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TC7116/A/TC7117/A

DS21457C-page 12

4.0

COMPONENT VALUE

SELECTION

4.1

Auto-Zero Capacitor

The size of the auto-zero capacitor has some influence

on system noise. For 200mV full scale, where noise is

very important, a 0.47

μF capacitor is recommended.

On the 2V scale, a 0.047

μF capacitor increases the

speed of recovery from overload and is adequate for

noise on this scale.

4.2

Reference Capacitor

A 0.1

μF capacitor is acceptable in most applications.

However, where a large Common mode voltage exists

(i.e., the VIN- pin is not at analog common), and a

200mV scale is used, a larger value is required to pre-

vent rollover error. Generally, 1

μF will hold the rollover

error to 0.5 count in this instance.

4.3

Integrating Capacitor

The integrating capacitor should be selected to give the

maximum voltage swing that ensures tolerance buildup

will not saturate the integrator swing (approximately

0.3V from either supply). In the TC7116/TC7116A or

the TC7117/TC7117A, when the analog common is

used as a reference, a nominal ±2V full scale integrator

swing is acceptable. For the TC7117/TC7117A, with

±5V supplies and analog common tied to supply

ground, a ±3.5V to ±4V swing is nominal. For 3 read-

ings per second (48kHz clock), nominal values for CINT

are 0.22

μ1F and 0.10μF, respectively. If different oscil-

lator frequencies are used, these values should be

changed in inverse proportion to maintain the output

swing. The integrating capacitor must have low dielec-

tric absorption to prevent rollover errors. Polypropylene

capacitors are recommended for this application.

4.4

Integrating Resistor

Both the buffer amplifier and the integrator have a class

A output stage with 100

μA of quiescent current. They

can supply 20

μA of drive current with negligible non-

linearity. The integrating resistor should be large

enough to remain in this very linear region over the

input voltage range, but small enough that undue leak-

age requirements are not placed on the PC board. For

2V full scale, 470k

Ω is near optimum and, similarly,

47k

Ω for 200mV full scale.

4.5

Oscillator Components

For all frequency ranges, a 100k

Ω resistor is recom-

mended; the capacitor is selected from the equation:

EQUATION 4-1:

For a 48kHz clock (3 readings per second), C = 100pF.

4.6

Reference Voltage

To generate full scale output (2000 counts), the analog

input requirement is VIN = 2VREF. Thus, for the 200mV

and 2V scale, VREF should equal 100mV and 1V,

respectively. In many applications, where the ADC is

connected to a transducer, a scale factor exists

between the input voltage and the digital reading. For

instance, in a measuring system, the designer might like

to have a full scale reading when the voltage from the

transducer is 700mV. Instead of dividing the input down

to 200mV, the designer should use the input voltage

directly and select VREF = 350mV. Suitable values for

integrating resistor and capacitor would be 120kW and

0.22

μF. This makes the system slightly quieter and also

avoids a divider network on the input. The TC7117/

TC7117A, with ±5V supplies, can accept input signals

up to ±4V. Another advantage of this system is when a

digital reading of zero is desired for VIN ≠ 0. Tempera-

ture and weighing systems with a variable tare are

examples. This offset reading can be conveniently gen-

erated by connecting the voltage transducer between

VIN+ and analog common, and the variable (or fixed)

offset voltage between analog common and VIN-.

0.45

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