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5/25/01

SPT7855

equal:

equal:

Figure 4 shows an example of expected voltage drops for

ANALOG INPUT

with respect to the voltage reference. (See voltage refer-

ence section.)

The drive requirements for the analog inputs are very

minimal when compared to most other converters due to

the SPT7855’s extremely low input capacitance of only

5 pF and very high input resistance of 50 k

Ω.

The analog input should be protected through a series

resistor and diode clamping circuit as shown in figure 5.

Upon powerup, the SPT7855 begins its calibration algo-

rithm. In order to achieve the calibration accuracy re-

quired, the offset and gain adjustment step size is a frac-

tion of a 10-bit LSB. Since the calibration algorithm is an

oversampling process, a minimum of 10,000 clock cycles

are required. This results in a minimum calibration time

upon powerup of 400 µsec (for a 25 MHz clock). Once

calibrated, the SPT7855 remains calibrated over time and

temperature.

Since the calibration cycles are initiated on the rising edge

of the clock, the clock must be continuously applied for the

SPT7855 to remain in calibration.

INPUT PROTECTION

All I/O pads are protected with an on-chip protection

circuit shown in figure 6. This circuit provides ESD robust-

ness to 3.5 kV and prevents latch-up under severe dis-

charge conditions without degrading analog transition

times.

Figure 5 – Recommended Input Protection Circuit

47 W

D1

D2

ADC

Buffer

+V

–V

D1 = D2 = Hewlett-Packard HP5712 or equivalent

Figure 6 – On-Chip Protection Circuit

Analog

Pad

120 W

120 W

CALIBRATION

The SPT7855 uses an auto-calibration scheme to ensure

10-bit accuracy over time and temperature. Gain and off-

set errors are continually adjusted to 10-bit accuracy

during device operation. This process is completely trans-

parent to the user.

POWER SUPPLY SEQUENCING CONSIDERATIONS

All logic inputs should be held low until power to the device

has settled to the specific tolerances. Avoid power decou-

pling networks with large time constants that could delay

CLOCK INPUT

The SPT7855 is driven from a single-ended TTL-input

clock. Because the pipelined architecture operates on the

rising edge of the clock input, the device can operate over

a wide range of input clock duty cycles without degrading

the dynamic performance.