ADS574 datasheet(13/17 Pages) TI1

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ADS574

ance node as compared with traditional ADC574 architec-

tures, where the resistor divider network looks into a com-

parator input node at virtual ground.

To understand how this circuit works, it is necessary to

know that the input range on the internal sampling capacitor

is from 0V to +3.33V, and the analog input to the ADS574

must be converted to this range. Unipolar 20V range can be

used as an example of how the divider network functions. In

20V operation, the analog input goes into pin 14. Pin 13 is

left unconnected and pin 12 is connected to analog common

pin 9. From Figure 12, it is clear that the input to the

capacitor array will be the analog input voltage on pin 14

divided by the resistor network (68k

Ω + 68kΩ || 17kΩ). A

20V input at pin 14 is divided to 3.33V at the capacitor

array, while a 0V input at pin 14 gives 0V at the capacitor

array.

The main effect of the 10k

Ω internal resistor on pin 12 is to

provide offset adjust response the same as that of traditional

ADC574 architectures without needing to change the exter-

nal trimpot values.

SINGLE SUPPLY OPERATION

The ADS574 is designed to operate from a single +5V

supply, and handle all of the unipolar and bipolar input

ranges, in either the Control Mode or the Emulation Mode as

described above. Pin 7 is not connected internally. This is

where +12V or +15V is supplied on traditional ADC574s.

Pin 11, the –12V or –15V supply input on traditional

ADC574s, is used only as a logic input on the ADS574.

There is a resistor divider internally on pin 11 to reduce that

input to a correct logic level within the ADS574, and this

resistor will add 10mW to 15mW to the power consumption

of the ADS574 when –15V is supplied to pin 11. To

minimize power consumption in a system, pin 11 can be

simply grounded (for Emulation Mode) or tied to +5V (for

Control Mode.)

There are no other modifications required for the ADS574 to

function with a single +5V supply.

CALIBRATION

OPTIONAL EXTERNAL FULL-SCALE

AND OFFSET ADJUSTMENTS

Offset and full-scale errors may be trimmed to zero using

external offset and full-scale trim potentiometers connected

to the ADS574 as shown in Figures 10 and 11 for unipolar

and bipolar operation.

CALIBRATION PROCEDURE—

UNIPOLAR RANGES

If external adjustments of full-scale and offset are not

required, replace R

2 in Figure 10 with a 50Ω, 1% metal film

resistor, omitting the other adjustment components. Connect

pin 12 to pin 9.

If adjustment is required, connect the converter as shown in

Figure 10. Sweep the input through the end-point transition

voltage (0V + 1/2LSB; +1.22mV for the 10V range, +2.44mV

for the 20V range) that causes the output code to be DB0 ON

(HIGH). Adjust potentiometer R1 until DB0 is alternately

toggling ON and OFF with all other bits OFF. Then adjust

full scale by applying an input voltage of nominal full-scale

minus 3/2LSB, the value which should cause all bits to be

ON. This value is +9.9963V for the 10V range and +19.9927V

for the 20V range. Adjust potentiometer R2 until bits DB1-

DB11 are ON and DB0 is toggling ON and OFF.

CALIBRATION PROCEDURE—BIPOLAR RANGES

If external adjustments of full-scale and bipolar offset are

not required, replace the potentiometers in Figure 11 by

Ω, 1% metal film resistors.

If adjustments are required, connect the converter as shown

in Figure 11. The calibration procedure is similar to that

described above for unipolar operation, except that the offset

adjustment is performed with an input voltage which is

1/2LSB above the minus full-scale value (–4.9988V for the

±5V range, –9.9976V for the ±10V range). Adjust R

1 for

DB0 to toggle ON and OFF with all other bits OFF. To

adjust full-scale, apply a DC input signal which is 3/2LSB

below the nominal plus full-scale value (+4.9963V for

±5V

range, +9.9927V for

±10V range) and adjust R

2 for DB0 to

toggle ON and OFF with all other bits ON.

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