AD5533 データシートの表示（PDF） - Analog Devices

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AD5533

32-Channel Infinite Sample-and-Hold

Analog Devices 

AD5533

FUNCTIONAL DESCRIPTION

The AD5533 can be thought of as consisting of an ADC and

32 DACs in a single package. The input voltage VIN 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 (VOUT0–VOUT31). Since

the channel output voltage is effectively the output of a DAC,

there is no droop associated with it. As long as power to the

device is maintained, 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, VIN. The desired

channel is then addressed via the Parallel Port or the Serial Port.

When the channel address has been loaded, provided TRACK is

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

DAC in order that the DAC output matches the voltage on VIN.

The BUSY Pin goes low and remains so until the acquisition is

complete. The noninverting input to the output buffer is tied to VIN

during the acquisition period to avoid spurious outputs, while

the DAC acquires the correct code. The acquisition is completed

in 16 µs 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 VIN in ISHA Mode is 70 mV. The maximum

VIN is 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 VOUT0 to VOUT31 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 = > VOUT = (Gain × VDAC) – (Gain – 1) × VOFFS_IN = 50 mV.

Analog Input

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

Capacitor C1 is typically 20 pF and can be attributed to the pin

capacitance 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

1 µs–2 µs of channel selection so that VIN can be acquired accurately.

For this reason a low impedance source is recommended.

VIN

C1

20pF

ADDRESSED CHANNEL

C2

7.5pF

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

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 the OFFS_IN Pin.

VOUT = 3.52 × VDAC − 2.52 × VOFFS _ IN

VDAC is the output of the DAC.

VOFFS_IN is the voltage at the OFFS_IN Pin.

Table I shows how the output range on VOUT relates to the offset

voltage supplied by the user.

Table I. Sample Output Voltage Ranges

VOFFS_IN (V)

0

1

2.130

VDAC (V)

0.05 to 3

0.05 to 3

0.05 to 3

VOUT (V)

0.176 to 10.56

–2.34 to +8.06

–5.192 to +5.192

VOUT is limited only by the headroom of the output amplifiers,

VOUT must be within the 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 VIN and acquired by the offset DAC. This offset channel’s

DAC output is directly connected to OFFS_OUT. By connect-

ing 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 VOUT is within maximum ratings.

CONTROLLER

VIN

DAC

BUSY

TRACK

ACQUISITION

CIRCUIT

OUTPUT

STAGE

AD5533

ONLY ONE CHANNEL SHOWN FOR SIMPLICITY

PIN

DRIVER

VOUT1

THRESHOLD

VOLTAGE

Figure 7. Typical ATE Circuit Using TRACK Input

DEVICE

UNDER

TEST

REV. A

–11–

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