8

Detailed Description

Theory of Operation

The HI5630 is a triple 8-Bit fully differential sampling pipeline

A/D converter with digital error correction logic. Each of the

three channels are identical so this discussion will only cover

one channel. Figure 3 depicts the circuit for the front end

differential-in-differential-out sample-and-hold (S/H). The

switches are controlled by an internal sampling clock which

is a non-overlapping two phase signal,

Φ

from the master sampling clock. During the sampling phase,

Φ

discharged to analog ground. At the falling edge of

Φ

input signal is sampled on the bottom plates of the sampling

capacitors. In the next clock phase,

Φ

plates of the sampling capacitors are connected together

and the holding capacitors are switched to the op amp

end sample-and-hold output is a fully-differential, sampled-

data representation of the analog input. The circuit not only

performs the sample-and-hold function but will also convert

a single-ended input to a fully-differential output for the

small values of these components result in a typical full

power input bandwidth of 250MHz for the converter.

As illustrated in the functional block diagram and the timing

diagram, identical pipeline subconverter stages, each

containing a two-bit flash converter and a two-bit multiplying

digital-to-analog converter, follow the S/H circuit with the last

stage being a two bit flash converter. Each converter stage

in the pipeline will be sampling in one phase and amplifying

in the other clock phase. Each individual subconverter clock

signal is offset by 180 degrees from the previous stage clock

signal resulting in alternate stages in the pipeline performing

the same operation.

The output of each of the identical two-bit subconverter

stages is a two-bit digital word containing a supplementary

bit to be used by the digital error correction logic. The output

of each subconverter stage is input to a digital delay line

which is controlled by the internal sampling clock. The

function of the digital delay line is to time align the digital

outputs of the identical two-bit subconverter stages with the

corresponding output of the last stage flash converter before

applying the results to the digital error correction logic. The

digital error correction logic uses the supplementary bits to

correct any error that may exist before generating the final

ten bit digital data output of the converter.

Because of the pipeline nature of this converter, the digital

data representing an analog input sample is output to the

digital data bus on the 5th cycle of the clock after the analog

sample is taken. This time delay is specified as the data

latency. After the data latency time, the digital data

representing each succeeding analog sample is output

during the following clock cycle. The digital output data is

synchronized to the external sampling clock by a double

buffered latching technique. The digital output data is

available in two’s complement or offset binary format

depending on the state of the Data Format Select (DFS)

control input (see Table 1, A/D Code Table).

The HI5630 is equipped with an internal reference voltage

generator, therefore, no external reference voltage is

internal reference voltage. An internal band-gap reference

voltage followed by an amplifier/buffer generates the

precision +2.5V reference voltage used by the converter. A

two-stage op amp closes the loop to create an internal

+1.25V band-gap reference voltage. This voltage is then

amplified by a wide-band uncompensated operational

amplifier connected in a gain-of-two configuration. An

external, user-supplied, 1

µF capacitor connected from the

pole and to maintain the stability of the operational amplifier.

The HI5630 is designed to accept a +2.5V reference voltage

differential analog input voltage range of

±0.5V.

The user does have the option of supplying an external

+2.5V reference voltage. As a result of the high input

the external reference voltage being used is only required to

source 1mA of reference input current. In the situation where

an external reference voltage will be used an external 1

µF

analog ground in order to maintain the stability of the internal

operational amplifier.

In order to minimize overall converter noise it is

recommended that adequate high frequency decoupling be

-

+

+

-

Φ

1

Φ

1

Φ

1

Φ

2

Φ

1

Φ

1

Φ

1

FIGURE 3. ANALOG INPUT SAMPLE-AND-HOLD

HI5630