__________Applications Information

Power-On Reset

At power-up, the internal power-supply circuitry sets INT

high and puts the device in normal operation / external

clock mode. This state is selected to keep the internal

clock from loading the external clock driver when the

part is used in external clock mode.

Internal or External Reference

The MAX199 can operate with either an internal or external

reference. An external reference can be connected to

either the REF pin or to the REFADJ pin (Figure 9).

To use the REF input directly, disable the internal buffer

nates the need to buffer the reference externally.

When the reference is applied at REFADJ, bypass

REFADJ with a 0.01µF capacitor to AGND.

The REFADJ internal buffer gain is trimmed to 1.6384 to

provide 4.096V at the REF pin from a 2.5V reference.

Internal Reference

The internally trimmed 2.50V reference is gained

through the REFADJ buffer to provide 4.096V at REF.

Bypass the REF pin with a 4.7µF capacitor to AGND

and the REFADJ pin with a 0.01µF capacitor to AGND.

The internal reference voltage is adjustable to ±1.5%

(±65 LSBs) with the reference-adjust circuit of Figure 1.

External Reference

At REF and REFADJ, the input impedance is a mini-

mum of 10k

Ω for DC currents. During conversions, an

external reference at REF must be able to deliver

400µA DC load currents, and must have an output

impedance of 10

Ω or less. If the reference has higher

input impedance or is noisy, bypass it close to the REF

pin with a 4.7µF capacitor to AGND.

With an external reference voltage of less than 4.096V

at the REF pin or less than 2.5V at the REFADJ pin, the

increase in the ratio of the RMS noise to the LSB value

(FS / 4096) results in performance degradation (loss of

effective bits).

Multi-Range (±4V, ±2V, +4V, +2V),

+5V Supply, 12-Bit DAS with 8+4 Bus Interface

______________________________________________________________________________________

13

REF

10k

2.5V

26 4.096V

4.7

µF

0.01

µF

25

REFADJ

MAX199

Figure 9a. Internal Reference

REF

10k

2.5V

26 4.096V

4.7

µF

2.5V

25

REFADJ

0.01

µF

MAX199

Figure 9c. The external reference at REFADJ overdrives the

internal reference.

REF

10k

2.5V

26

4.096V

4.7

µF

25

REFADJ

MAX199

Figure 9b. External Reference at REF