ADS774 datasheet(12/21 Pages) TI2

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ADS774

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 ADS774 is typically 50k

Ω in the

20V ranges and 12k

Ω in the 10V ranges. This is signifi-

cantly higher than that of traditional ADC774 architectures,

reducing the load on the input source in most applications.

INPUT STRUCTURE

Figure 12 shows the resistor divider input structure of the

ADS774. Since the input is driving a capacitor in the CDAC

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

ance node as compared with traditional ADC774 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 ADS774

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 pin 9, analog

common. 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 (42k

Ω + 42kΩ || 10.5kΩ). 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 the same offset adjust response as that of traditional

ADC774 architectures without changing the external trimpot

values.

SINGLE SUPPLY OPERATION

The ADS774 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

FIGURE 10. Unipolar Configuration.

100

Ω

100

Ω

Analog

Common

100k

Ω

100k

Ω

–V

Unipolar

Offset

Adjust

Full-Scale

Adjust

Ref In

Ref Out

Bipolar Offset

ADS774

10V

Range

Analog

Input

20V

Range

2.5V

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 ADS774 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

100

Ω

10V

Range

Analog

Input

Analog

Common

Full-Scale Adjust

Ref In

Ref Out

Bipolar Offset

ADS774

20V

Range

Bipolar

Offset

Adjust

100

Ω

2.5V

FIGURE 11. Bipolar Configuration.

FIGURE 12. ADS774 Input Structure.

Capacitor

Array*

42k

Ω

21k

Ω

10.5k

Ω

10k

Ω

21k

Ω

Pin 12

Pin 13

Pin 14

20V Range

10V Range

Bipolar

Offset

*10pF when sampling

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