ADS807

13

SBAS072A

www.ti.com

REFERENCE OPERATION

The internal reference consists of a bandgap voltage refer-

ence, the drivers for the top and bottom reference, and the

resistive reference ladder. The bandgap reference circuit

includes logic functions that allow setting the analog input

swing of the ADS807 to a differential full-scale range of either

HIGH potential, respectively. While operating the ADS807 in

the external reference mode, the buffer amplifiers for the

REFT and REFB are disabled. The ADS807 has an internal

50k

Ω pull-down resistor at the range select pin (RSEL).

Therefore, this pin can be either hardwired to ground or left

unconnected, which will default the converter to a 2Vp-p full-

scale input range (FSR). While set for the 2Vp-p range, the

top and bottom reference voltages will be REFT = +3.0V and

REFB = +2.0V. Switching to the 3Vp-p range changes those

voltages to REFT = +3.25V and REFB = +1.75V. The

reference buffers can be utilized to supply up to 1mA (sink

and source) to external circuitry. To ensure proper operation

with any reference configuration, it is necessary to provide

solid bypassing at all reference pins in order to keep the

clock feedthrough to a minimum, as shown in Figure 6. Good

performance requires using 0.1

µF low inductance capaci-

tors. All bypassing capacitors should be located as close to

their respective pins as possible.

USING EXTERNAL REFERENCES

For even more design flexibility, the internal reference can

be disabled and an external reference voltage used. The

utilization of an external reference may be considered for

applications requiring higher accuracy, improved tempera-

ture performance, or a wide adjustment range of the

converter’s full-scale range. In multichannel applications, the

use of a common external reference has the benefit of

obtaining better matching and drift of the full-scale range

between converters. Figure 7 gives an example of an exter-

nal reference circuit using a single-supply, low-power, dual

op amp (OPA2234).

The external references can vary as long as the value of the

external top reference (REFT EXT) stays within the range of

reference (REFB EXT) stays within 1.70V and REFT – 0.4V.

disabled while the converter operates in external reference

mode. Setting the ADS807 for external reference mode re-

quires the INT/EXT pin (pin 18) to be HIGH.

The logic level applied to the INT/EXT pin of the ADS807

determines if the converter operates with either the built-in

reference or external reference voltages. Because this function

pin has an internal 50k

Ω pull-up resistor, the default configu-

ration is external reference mode. Grounding this pin will

activate the internal reference option.

The input track-and-hold amplifier is differential. A positive

1Vp-p on the IN and its compliment, a negative

1Vp-p, on the IN (see Figure 3) results in 2Vp-p on the

output of the track-and-hold. Likewise, 2Vp-p on the IN and

0Vp-p on the IN (see Figure 4) results in 2Vp-p on the

output of the track-and-hold. Therefore, the reference volt-

ages, REFT and REFB, are the same for both differential

and single-ended inputs, see Table I.

The external references may be changed for different tasks.

The ADS807 will follow the external references with a latency

to change the configuration of a circuit for different tasks, a

large amount of time must be allowed. This time could be

hundreds of microseconds. Refer to the diagram on the front

page. Note that there is no disconnect for external references.

FIGURE 7. Example for an External Reference Driver Using the Dual, Single-Supply Op Amp, OPA2234.

REFT

CM

0.1

µF

(1)

(1)

(1)

REFB

ADS807

0.1

µF

0.1

µF

10

µF

NOTE: (1) Optional.

+

10

µF

+

10

µF

+

FIGURE 6. Recommended Bypassing for the Reference Pins.

OPA2234

A1

OPA2234

A2

R

3

R

4

R

2

R

1

+5V

+5V

0.1µF

10µF

> 1.70V

< 3.30V

Top Reference

Bottom Reference

+

4.7k

Ω

REF1004

+2.5V