TC850 datasheet(15/30 Pages) MICROCHIP

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 2001-2012 Microchip Technology Inc.

DS21479D-page 15

TC850

7.0

ANALOG SECTION TYPICAL

APPLICATIONS

7.1

Component Selection

7.1.1

REFERENCE VOLTAGE

The typical value for reference voltage VREF1 is

1.6384V. This value yields a full scale voltage of

3.2768V and resolution of 100

V per step. The V

REF2

value is derived by dividing VREF1 by 64. Thus, typical

VREF2 value is 1.6384V/64, or 25.6 mV. The VREF2

value should be adjusted within ±1% to maintain 15-bit

accuracy for the total conversion process;

EQUATION 7-1:

The reference voltage is not limited to exactly 1.6384V,

however, because the TC850 performs a ratiometric

conversion. Therefore, the conversion result will be:

EQUATION 7-2:

The full scale voltage can range from 3.2V to 3.5V. Full

scale voltages of less than 3.2V will result in increased

noise in the Least Significant bits, while a full scale

above 3.5V will exceed the input common-mode range.

7.1.2

INTEGRATION RESISTOR

The TC850 buffer supplies 25

A of integrator charging

current with minimal linearity error. RINT is easily

calculated:

EQUATION 7-3:

For a full scale voltage of 3.2768V, values of RINT

between 120 k

 and 150 k are acceptable.

7.1.3

INTEGRATION CAPACITOR

The integration capacitor should be selected to

produce an integrator swing of

4V at full scale. The

capacitor value is easily calculated:

EQUATION 7-4:

The integration capacitor should be selected for low

dielectric absorption to prevent rollover errors. A poly-

propylene, polyester or polycarbonate dielectric capac-

itor is recommended.

7.1.4

REFERENCE CAPACITORS

The reference capacitors require a low-leakage dielec-

tric, such as polypropylene, polyester or polycarbon-

ate. A value of 1

F is recommended for operation over

the temperature range. If high-temperature operation is

not required, the CREF values can be reduced.

7.1.5

AUTO-ZERO CAPACITORS

Five capacitors are required to auto-zero the input buf-

fer, integrator amplifier and comparator. Recom-

mended capacitors are 0.1

F film dielectric (such as

polyester or polypropylene). Ceramic capacitors are

not recommended.

VREF =

VREF1 ± 1%

Digital Counts =

• 16384

VIN

VREF1

RINT =

VFULLSCALE

A

C =

VFS

RINT

•

4 • 256

4V FCLOCK

where:

FCLOCK is the crystal or external oscillator

frequency and VFS is the maximum input voltage.

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