EVAL-AD5533EB【PDF 11/16 页】32-Channel Precision Infinite Sample-and

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REV. A

AD5533B

–11–

TRACK

DAC

ACQUISITION

CIRCUIT

OUT

BUSY

STAGE

CONTROLLER

THRESHOLD

VOLTAGE

DRIVER

DEVICE

UNDER

TEST

ONLY ONE CHANNEL SHOWN FOR SIMPLICITY

AD5533B

OUTPUT

Figure 7. Typical ATE Circuit Using

TRACK

Input

FUNCTIONAL DESCRIPTION

The AD5533B can be thought of as consisting of an ADC and

32 DACs in a single package. The input voltage V

is sampled

and converted into a digital word. The digital result is loaded into

one of the DAC registers and is converted (with gain and offset)

into an analog output voltage (V

OUT

0–V

OUT

31). Since the chan-

nel output voltage is effectively the output of a DAC there is no

droop associated with it. As long as power to the device is main-

tained, the output voltage will remain constant until this channel

is addressed again.

To update a single channel’s output voltage, the required new

voltage level is set up on the common input pin, V

. The desired

channel is then addressed via the parallel port or the serial port.

When the channel address has been loaded, provided

TRACK

high, the circuit begins to acquire the correct code to load to the

DAC so that the DAC output matches the voltage on V

. The

BUSY

pin goes low and remains so until the acquisition is com-

plete. The noninverting input to the output buffer is tied to V

during the acquisition period to avoid spurious outputs while the

DAC acquires the correct code. The acquisition is completed in

s max. The

BUSY

pin goes high and the updated DAC output

assumes control of the output voltage. The output voltage of the

DAC is connected to the noninverting input of the output buffer.

Since the internal DACs are offset by 70 mV (max) from GND,

the minimum V

in ISHA mode is 70 mV. The maximum V

2.96 V due to the upper dead band of 40 mV (max).

On power-on, all the DACs, including the offset channel, are loaded

with zeros. Each of the 33 DACs is offset internally by 50 mV (typ)

from GND so the outputs V

OUT

0 to V

OUT

31 are 50 mV (typ) on

power-on if the OFFS_IN pin is driven directly by the on-board

offset channel (OFFS_OUT), i.e., if OFFS_IN = OFFS_OUT =

50 mV = > V

OUT

= (Gain

DAC

)

– (Gain – 1)

Analog Input

The equivalent analog input circuit is shown in Figure 6. The

capacitor C1 is typically 20 pF and can be attributed to pin capaci-

tance and 32 off-channels. When a channel is selected, an extra

7.5 pF (typ) is switched in. This capacitor C2 is charged to the

previously acquired voltage on that particular channel so it must

charge/discharge to the new level. It is essential that the external

source can charge/discharge this additional capacitance within

s–2

s of channel selection so that V

can be acquired accu-

rately. For this reason, a low impedance source is recommended.

OFFS_IN

= 50 mV.

20pF

7.5pF

ADDRESSED CHANNEL

Figure 6. Analog Input Circuit

Large source impedances will significantly affect the performance

of the ADC. This may necessitate the use of an input buffer

amplifier.

Output Buffer Stage—Gain and Offset

The function of the output buffer stage is to translate the

50 mV–3 V typical output of the DAC to a wider range. This

is done by gaining up the DAC output by 3.52 and offsetting

the voltage by the voltage on OFFS_IN pin.

DAC

is the output of the DAC.

OFFS_IN

is the voltage at the OFFS_IN pin.

Table I shows how the output range on V

OUT

relates to the

offset voltage supplied by the user.

3.52 V

2.52 V

OUT

DAC

OFFS_IN

Table I. Sample Output Voltage Ranges

OFFS_IN

(V)

2.130

DAC

(V)

0.05 to 3

OUT

(V)

0.176 to 10.56

–2.34 to +8.04

–5.192 to +5.192

OUT

is limited only by the headroom of the output amplifiers.

OUT

must be within maximum ratings.

Offset Voltage Channel

The offset voltage can be externally supplied by the user at

OFFS_IN or it can be supplied by an additional offset voltage

channel on the device itself. The required offset voltage is set up

on V

and acquired by the offset DAC. This offset channel’s DAC

output is directly connected to OFFS_OUT. By connecting

OFFS_OUT to OFFS_IN, this offset voltage can be used as the

offset voltage for the 32 output amplifiers. It is important to

choose the offset so that V

OUT

is within maximum ratings.

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