CS5516-AS datasheet(35/41 Pages) CIRRUS

35 / 41 page

ample, the board comes with a set of precision

resistors which divide the excitation supply

(nominally 10 volts total) down to 2.5 volts be-

tween the VREF+ and VREF- input pins. This

sets the nominal full scale voltage into the A/D

converter. The input span of the instrumentation

amplifier can be calculated to by knowing the

PGA gain setting, and that the gain of the instru-

mentation amplifier is X25. If the PGA is set for

a gain of 8, then the input span to the instrumen-

tation amplifier will be 2.5 volts (VREF+ -

VREF-) divided by 8 X 25, or 2.5/(200) = 12.5

millivolt nominal in unipolar mode. The device

can be then calibrated with an input voltage

which is as low as 20% less than nominal or up

to 20% greater than nominal.

Therefore, with

this VREF+ - VREF- voltage (2.5 volts) and a

PGA gain of 8 the input span can be calibrated

to handle a span from a low of 10 mV to a high

of 15 mV. To modify the input span the user can

either change the PGA gain or modify the resis-

tor divider on the bridge sense voltage to yield

an appropriate value in the range of 2.0 to 3.8

volts. This makes the A/D converter quite flex-

ible in handling load cells with different output

levels.

Whenever configured as a bridge

transducer device, the CS5516 or the CS5520

A/D converter operates in ratiometric measure-

ment mode. Figures 4 and 5 illustrate how to

connect 4-wire and 6-wire bridge transducers to

the board.

Alternatively, the CS5516 or CS5520 can be

configured for absolute measurement if a preci-

sion reference voltage is supplied between the

VREF+ and VREF- pins of the A/D converter.

The board can be modified to accept a reference

into the voltage reference terminal block; or the

on-board LT1019-2.5 volt reference can be used

as the reference voltage for the A/D converter.

To use either of these inputs will require that

jumper wires be soldered in either 1A-1B to se-

lect the external voltage reference input, or

2A-2B to select the on-board LT1019-2.5. Fig-

ure 6 illustrates the connection of an external

voltage reference to the evaluation board for ab-

solute voltage measurement applications. To

achieve an accurate reference voltage resistor R6

SIG +

SIG -

SENSE +

SENSE -

EXC +

EXC -

Figure 4. 4-Wire Bridge Connections

SIG +

SIG -

SENSE +

SENSE -

EXC +

EXC -

Figure 5. 6-Wire Bridge Connections

CDB5516/CDB5520

DS74DB3

DS74DB4

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