TC7117ACPL datasheet(11/14 Pages) TELCOM

11 / 14 page

3-213

TELCOM SEMICONDUCTOR, INC.

3-1/2 DIGIT ANALOG-TO-DIGITAL

CONVERTERS WITH HOLD

TC7116

TC7116A

TC7117

TC7117A

COMPONENT VALUE SELECTION

Auto-Zero Capacitor

The size of the auto-zero capacitor has some influ-

ence on system noise. For 200 mV 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.

Reference Capacitor

A 0.1

µF capacitor is acceptable in most applications.

However, where a large common-mode voltage exists (i.e.,

the V

–

IN pin is not at analog common), and a 200-mV scale

is used, a larger value is required to prevent roll-over error.

Generally, 1

µF will hold the roll-over error to 0.5 count in

this instance.

Integrating Capacitor

The integrating capacitor should be selected to give the

maximum voltage swing that ensures tolerance build-up 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 com-

mon tied to supply ground, a

±3.5V to ±4V swing is nominal.

For 3 readings per second (48 kHz clock), nominal values

for CINT are 0.22 µ1F and 0.10 µF, respectively. If different

oscillator frequencies are used, these values should be

changed in inverse proportion to maintain the output swing.

The integrating capacitor must have low dielectric ab-

sorption to prevent roll-over errors. Polypropylene capaci-

tors are recommended for this application.

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 nonlinearity.

The integrating resistor should be large enough to remain

in this very linear region over the input voltage range, but

small enough that undue leakage requirements are not

placed on the PC board. For 2V full scale, 470 k

Ω is near

optimum and, similarly, 47 k

Ω for 200 mV full scale.

Oscillator Components

For all frequency ranges, a 100-k

Ω resistor is recom-

mended; the capacitor is selected from the equation:

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

Figure 10. Negative Power Supply Generation With TC7660

VREF

TC7117

TC7117A

10 µF

VIN

–

COM

GND

10 µF

VIN

–

(–5V)

+5V

TC7660

–

TC04

LED

DRIVE

Reference Voltage

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

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

2V scale, VREF should equal 100 mV 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 700 mV. Instead of dividing

the input down to 200 mV, the designer should use the input

voltage directly and select VREF = 350 mV. Suitable values

for integrating resistor and capacitor would be 120 k

Ω 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. Temperature and weighing

systems with a variable tare are examples. This offset

reading can be conveniently generated by connecting the

voltage transducer between V

IN and analog common, and

the variable (or fixed) offset voltage between analog com-

mon and V

–

IN.

TC7117/TC7117A POWER SUPPLIES

The TC7117/TC7117A are designed to operate from

±5V supplies. However, if a negative supply is not available,

it can be generated with a TC7660 DC-to-DC converter and

two capacitors. Figure 10 shows this application.

In selected applications, a negative supply is not re-

quired. The conditions for using a single +5V supply are:

(1) The input signal can be referenced to the center of

the common-mode range of the converter.

(2) The signal is less than

±1.5V.

(3) An external reference is used.

f =

TC7117ACPL Datasheet(PDF) 11 Page - TelCom Semiconductor, Inc

TC7117ACPL Datasheet(HTML) 11 Page - TelCom Semiconductor, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14

Datasheet Download

Link URL