AD688

REV. A

–5–

If further noise reduction is desired, an optional capacitor may

be added between the NOISE REDUCTION pin and ground as

shown in Figure 2b. This will form a low pass filter with the

5k

Ω R

µF capacitor will

have a 3 dB point at 32 Hz and will reduce the high frequency

noise (to 1 MHz) to about 250

µV p-p. Figure 4 shows the 1 MHz

noise of a typical AD688 both with and without a 1

µF capacitor.

Figure 4. Effect of 1

µF Noise Reduction Capacitor on

Broadband Noise

TURN-ON TIME

Upon application of power (cold start), the time required for the

output voltage to reach its final value within a specified error is

the turn-on settling time. Two components normally associated

with this are: time for active circuits to settle and time for ther-

mal gradients on the chip to stabilize. Figure 5 shows the turn-

on characteristics of the AD688. It shows the settling time to be

about 600

µs. Note the absence of any thermal tails when the

horizontal scale is expanded to 2 ms/cm in Figure 5b.

a. Electrical Turn-On

b. Extended Time Scale

Figure 5. Turn-On Characteristics

Output turn-on time is modified when an external noise reduc-

tion capacitor is used. When present, this capacitor presents an

additional load to the internal Zener diode’s current source,

resulting in a somewhat longer turn-on time. In the case of a

1

µF capacitor, the initial turn-on time is approximately 100 ms

(see Figure 6).

When the NOISE REDUCTION feature is used, a 20 k

Ω

resistor between Pins 6 and 2 is required for proper startup.

Figure 6. Turn-On With 1

µF C

N

TEMPERATURE PERFORMANCE

The AD688 is designed for precision reference applications

where temperature performance is critical. Extensive tempera-

ture testing ensures that the device’s high level of performance is

maintained over the operating temperature range.

Figure 7 shows the typical output voltage drift for the

AD688SQ and illustrates the test methodology. The box in Fig-

ure 7 is bounded on the sides by the operating temperature

extremes and on top and bottom by the maximum and

minimum +10 V output error voltages measured over the

operating temperature range. The slopes of the diagonals drawn

for both the +10 V and –10 V outputs determine the perform-

ance grade of the device.

Figure 7. Typical AD688SQ Temperature Drift

Each AD688A and B grade unit is tested at –40

°C, –25°C, 0°C,

+25

°C, +50°C, +70°C and +85°C. Each AD688S grade unit is

tested at –55

°C, –25°C, +25°C, +70°C and +125°C. This

approach ensures that the variations of output voltage that occur

as the temperature changes within the specified range will be

contained within a box whose diagonal has a slope equal to the

maximum specified drift. The position of the box on the vertical

scale will change from device to device as initial error and the

shape of the curve vary. Maximum height of the box for the

appropriate temperature range is shown in Figure 8.

Figure 8. Maximum +10 V or –10 V Output Change