ADS574 datasheet(12/17 Pages) TI1

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ADS574

POWER SUPPLY DECOUPLING

On the ADS574, +5V (to Pin 1) is the only power supply

required for correct operation. Pin 7 is not connected inter-

nally, so there is no problem in existing ADC574 sockets

where this is connected to +15V. Pin 11 (VEE) is only used

as a logic input to select modes of control over the sampling

function as described above. When used in an existing

ADC574 socket, the –15V on pin 11 selects the ADC574

Emulation Mode. Since pin 11 is used as a logic input, it is

immune to typical supply variations.

The +5V supply should be bypassed with a 10

µF tantalum

capacitor located close to the converter to promote noise-

free operations, as shown in Figure 2. Noise on the power

supply lines can degrade the converter’s performance. Noise

and spikes from a switching power supply are especially

troublesome.

RANGE CONNECTIONS

The ADS574 offers four standard input ranges: 0V to +10V,

0V to +20V,

±5V, or ±10V. Figures 10 and 11 show the

necessary connections for each of these ranges, along with

the optional gain and offset trim circuits. If a 10V input

range is required, the analog input signal should be con-

nected to pin 13 of the converter. A signal requiring a 20V

range is connected to pin 14. In either case the other pin of

the two is left unconnected. Pin 12 (Bipolar Offset) is

connected either to Pin 9 (Analog Common) for unipolar

operation, or to Pin 8 (2.5V Ref Out), or the external

reference, for bipolar operation. Full-scale and offset adjust-

ments are described below.

The input impedance of the ADS574 is typically 84k

Ω in the

20V ranges and 21k

Ω in the 10V ranges. This is signifi-

cantly higher than that of traditional ADC574 architectures,

reducing the load on the input source in most applications.

INPUT STRUCTURE

Figure 12 shows the resistor divider input structure of the

ADS574. Since the input is driving a capacitor in the CDAC

during acquisition, the input is looking into a high imped-

FIGURE 10. Unipolar Configuration.

FIGURE 11. Bipolar Configuration.

If the 10V analog input range is used (either bipolar or

unipolar), the 20V range input (pin 14) should be shielded

with ground plane to reduce noise pickup.

Coupling between analog input and digital lines should be

minimized by careful layout. For instance, if the lines must

cross, they should do so at right angles. Parallel analog and

digital lines should be separated from each other by a pattern

connected to common.

If external full scale and offset potentiometers are used, the

potentiometers and associated resistors should be as close as

possible to the ADS574.

100

Ω

10V

Range

Analog

Input

Analog

Common

Full-Scale Adjust

Ref In

Ref Out

Bipolar Offset

ADS574

20V

Range

Bipolar

Offset

Adjust

100

Ω

2.5V

FIGURE 12. ADS574 Input Structure.

Capacitor

Array

68k

Ω

34k

Ω

17k

Ω

10k

Ω

34k

Ω

Pin 12

Pin 13

Pin 14

20V Range

10V Range

Bipolar

Offset

100

Ω

100

Ω

Analog

Common

100k

Ω

100k

Ω

–V

Unipolar

Offset

Adjust

Full-Scale

Adjust

Ref In

Ref Out

Bipolar Offset

ADS574

10V

Range

Analog

Input

20V

Range

2.5V

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