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2/23/00

SPT7862

ANALOG INPUT

the respective input returns. Each input return is typically

tied to its respective low side reference ladder sense line.

(typically 4.0 V) and will scale proportionally with respect to

the voltage reference. (See the Voltage Reference section.)

The drive requirements for the analog inputs are very mini-

mal, when compared to most other converters, due to the

SPT7862’s extremely low input capacitance of only 5 pF

and a high input resistance in excess of 29 k

Ω.

Each analog input should be protected through a series

resistor and diode clamping circuit as shown in figure 5.

Figure 6 – On-Chip Protection Circuit

Analog

Pad

120

Ω

120

Ω

CLOCK INPUT

Each ADC is driven independently from a single-ended

TTL-input clock. Because the pipelined architecture oper-

ates on the rising edge of the clock input, each ADC can

operate over a wide range of input clock duty cycles without

degrading the dynamic performance.

DIGITAL OUTPUTS

The digital outputs (DA9–0 and DB9–0) are driven by sepa-

+5 V. This feature makes it possible to drive the SPT7862’s

TTL/CMOS-compatible outputs with the user’s logic system

supply. Each digital output supply may be driven indepen-

dently. The format of the output data (D0–D9) is straight

binary. (See Table III.) The outputs are latched on the rising

output ports. These outputs can be switched into a tri-state

Table III – Output Data Information

ANALOG INPUT

OVERRANGE

OUTPUT CODE

D10

D9–D0

+F.S. + 1/2 LSB

1

11 1111

1111

+F.S. –1/2 LSB

0

11 1111

111Ø

+1/2 F.S.

0

ØØ ØØØØ ØØØØ

+1/2 LSB

0

00 0000 000Ø

0.0 V

0

00 0000 0000

(Ø indicates the flickering bit between logic 0 and 1)

EVALUATION BOARD

The EB7862 evaluation board is available to aid designers

in demonstrating the full performance of the SPT7862.

This board includes a reference circuit, clock driver circuit,

output data latches and an on-board reconstruction of the

digital data. An application note describing the operation of

this board as well as information on the testing of the

SPT7862 is also available. Contact the factory for price and

availability.

Figure 5 – Recommended Input Protection Circuit

47

Ω

D1

D2

ADC

Buffer

+V

–V

D1 = D2 = Hewlett Packard HP5712 or equivalent

CALIBRATION

The SPT7862 uses a user-transparent, auto-calibration

scheme to ensure 10-bit accuracy over time and tempera-

ture. Gain and offset errors are continually adjusted to 10-bit

accuracy during device operation.

Upon power up, the SPT7862 begins its calibration algo-

rithm. In order to achieve the calibration accuracy required,

the offset and gain adjustment step size is a fraction 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 power up of

250

µsec (for a 40 MHz clock). Once calibrated, the

SPT7862 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

SPT7862 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 robustness and

prevents latch-up under severe discharge conditions with-

out degrading analog transition times.