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TELCOM SEMICONDUCTOR, INC.

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TC835

PERSONAL COMPUTER

DATA ACQUISITION A/D CONVERTER

The stability of the reference voltage is a major factor in

the overall absolute accuracy of the converter. For this

reason, it is recommended that a high-quality reference be

used where high-accuracy absolute measurements are

being made. Suitable references are:

Part Type

Manufacturer

TC04A

TelCom Semiconductor

TC9491

TelCom Semiconductor

APPLICATIONS INFORMATION

Component Value Selection

The integrating resistor is determined by the full-scale

input voltage and the output current of the buffer used to

charge the integrator capacitor. Both the buffer amplifier and

the integrator have a class A output stage, with 100

µA of

quiescent current. A 20

µA drive current gives negligible

linearity errors. Values of 5

µA to 40 µA give good results.

The exact value of an integrating resistor for a 20

µA current

is easily calculated.

Integrating Capacitor

The product of integrating resistor and capacitor should

be selected to give the maximum voltage swing that ensures

the tolerance buildup will not saturate the integrator swing

(approximately 0.3V from either supply). For

±5V supplies

and ANALOG COMMON tied to supply ground, a

±3.5V to

±4V full-scale integrator swing is adequate. A 0.10 µF to 0.47

=

[10,000

Integrator output voltage swing

(10,000) (clock period) (20

µA)

Integrator output voltage swing

A very important characteristic of the integrating capaci-

tor is that it has low dielectric absorption to prevent rollover

or ratiometric errors. A good test for dielectric absorption is

to use the capacitor with the input tied to the reference. This

ratiometric condition should read half-scale 0.9999, any

deviation is probably due to dielectric absorption. Polypro-

pylene capacitors give undetectable errors at reasonable

cost. Polystyrene and polycarbonate capacitors may also be

used in less critical applications.

Auto-Zero and Reference Capacitors

The size of the auto-zero capacitor has some influence

on the noise of the system. A large capacitor reduces the

noise. The reference capacitor should be large enough such

that stray capacitance to ground from its nodes is negligible.

The dielectric absorption of the reference capacitor and

auto-zero capacitor are only important at power-on, or when

the circuit is recovering from an overload. Smaller or cheaper

capacitors can be used if accurate readings are not required

for the first few seconds of recovery.

Reference Voltage

The analog input required to generate a full-scale output

Conversion Timing

Line Frequency Rejection

A signal integration period at a multiple of the 60 Hz line

frequency will maximize 60 Hz "line noise" rejection.

A 200 kHz clock frequency will reject 60 Hz and 400 Hz

noise. This corresponds to five readings per second.

full-scale voltage

20

µA

Conversion Rate vs Clock Frequency

Oscillator Frequency

Conversion Rate

(kHz)

(Conv/Sec)

100

2.5

120

3

200

5

300

7.5

400

10

800

20

1200

30

Oscillator Frequency

(kHz)

60 Hz

50 Hz

400 Hz

50.000

•

•

•

53.333

—

—

•

66.667

•

—

•

80.000

—

—

•

83.333

—

•

•

100.000

•

•

•

125.000

—

•

•

133.333

—

—

•

166.667

—

—

•

200.000

•

—

•

250.000

—

•

•

The conversion rate is easily calculated:

Conversion Rate

(Readings 1/sec) =

Line Frequency Rejection

Clock Frequency (Hz)

4000