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AD664

REV.

–6–

ANALOG CIRCUIT CONSIDERATIONS

Grounding Recommendations

The AD664 has two pins, designated ANALOG and DIGITAL

ground. The analog ground pin is the “high quality” ground ref-

erence point for the device. A unique internal design has

resulted in low analog ground current. This greatly simplifies

management of ground current and the associated induced volt-

age drops. The analog ground pin should be connected to the

analog ground point in the system. The external reference and

any external loads should also be returned to analog ground.

The digital ground pin should be connected to the digital

ground point in the circuit. This pin returns current from the

logic portions of the AD664 circuitry to ground.

Analog and digital grounds should be connected at one point in

the system. If there is a possibility that this connection be bro-

ken or otherwise disconnected, then two diodes should be con-

nected between the analog and digital ground pins of the

AD664 to limit the maximum ground voltage difference.

Power Supplies and Decoupling

The AD664 requires three power supplies for proper operation.

VLL powers the logic portions of the device and requires

+5 volts. VCC and VEE power the remaining portions of the cir-

cuitry and require +12 V to +15 V and –12 V to –15 V, respec-

tively. VCC and VEE must also be a minimum of two volts greater

then the maximum reference and output voltages anticipated.

Decoupling capacitors should be used on all power supply pins.

Good engineering practice dictates that the bypass capacitors be

located as near as possible to the package pins. VLL should be

bypassed to digital ground. VCC and VEE should be decoupled to

analog ground.

Driving the Reference Input

The reference input of the AD664 can have an impedance as

low as 1.3 k

Ω. Therefore, the external reference voltage must be

able to source up to 7.7 mA of load current. Suitable choices

include the 5 V AD586, the 10 V AD587 and the 8.192 V

AD689.

The architecture of the AD664 derives an inverted version of

the reference voltage for some portions of the internal circuitry.

This means that the power supplies must be at least 2 V

Figure 2. Recommended Circuit Schematic

greater than both the external reference and the inverted exter-

nal reference.

Output Considerations

Each DAC output can source or sink 5 mA of current to an

external load. Short-circuit protection limits load current to a

maximum load current of 40 mA. Load capacitance of up to

500 pF can be accommodated with no effect on stability.

Should an application require additional output current, a cur-

rent boosting element can be inserted into the output loop with

no sacrifice in accuracy. Figure 3 details this method.

Figure 3. Current-Boosting Scheme

AD664 output voltage settling time is 10

µs maximum. Figure 4

shows the output voltage settling time with a fixed 10 V refer-

ence, gain = 1 and all bits switched from 1 to 0.

Figure 4. Settling Time; All Bits Switched from On to Off

Alternately, Figure 5 shows the settling characteristics when the

reference is switched and the input bits remain fixed. In this

case, all bits are “on,” the gain is 1 and the reference is switched

from –5 V to +5 V.

Figure 5. Settling Time; Input Bits Fixed, Reference

Switched

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