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9/7/98

SPT7871

Figure 1 - Timing Diagram

THEORY OF OPERATION

The SPT7871 uses a two stage subranging architecture

incorporating a 3-bit flash MSB conversion stage followed by

an 8-bit interpolating folder conversion stage. Digital error

correction logic combines the results of both stages to pro-

duce a 10-bit data conversion digital output.

The analog signal is input directly to the 3-bit flash converter

which performs a 3-bit conversion and in turn drives an

internal DAC used to set the second stage voltage reference

level. The 3-bit result from the flash conversion is input to the

digital error correction logic and used in calculation of the

upper most significant bits of the data output.

The analog input is also input directly to an internal track-and-

hold amplifier. The signal is held and amplified for use in the

second stage conversion. The output of the track-and-hold is

input into a summing junction that takes the difference

between the track-and-hold amplifier and the 3-bit DAC

output. The residual is captured by a second track-and-hold

which holds and amplifies this residual voltage.

The residual held by the track-and-hold amplifier is input to an

8-bit interpolating folder stage for data conversion. The 8-bit

converted data from the folder stage is input into the digital

error correction logic and used in calculation of the lower

significant bits.

The error correction logic incorporates a proprietary scheme

for compensation of any internal offset and gain errors that

might exist to determine the 10-bit conversion result. The

resultant 10-bit data conversion is internally latched and

presented on the data output pins via buffered output drivers.

TYPICAL INTERFACE CIRCUIT

The SPT7871 requires few external components to achieve

the stated operation and performance. Figure 2 shows the

typical interface requirements when using the SPT7871 in

normal circuit operation. The following section is a description

of the pin functions and outlines critical performance criteria

to consider for achieving the optimal device performance.

POWER SUPPLIES AND GROUNDING

to the analog ground plane. Each power supply pin should be

bypassed as closely as possible to the device with .01

µF and

2.2

µF capacitors as shown in figure 2.

The two grounds available on the SPT7871 are AGND and

DGND. DGND is used only for TTL outputs and is to be

referenced to the output pullup voltage. These grounds are

not tied together internal to the device. The use of ground

planes is recommended to achieve the best performance of

the SPT7871. The AGND and the DGND ground planes

should be separated from each other and only connected

together at the device through an inductance or ferrite bead.

Doing this will minimize the ground noise pickup.

Table I - Data Output Timing Parameters

Timing Parameter

Minimum

Typical

Maximum

2 MHz

100 MHz

4.0 ns

250 ns

4.0 ns

250 ns

3 ns

Clock Latency

2 clock cycles

CLK

OUTPUT

DATA

DATA VALID

N

N

N+1

N+2

DATA VALID

N-1

N-3

N-2

tclk

tpwh

tpwl