SPT5510 datasheet(6/8 Pages) CADEKA

6 / 8 page

9/27/00

SPT5510

The feedback resistor should be matched to RSET to reduce

gain drift. Also, the op amp’s ground reference should be

the same as RSET’s to reduce gain and offset errors. A com-

posite amplifier may be required to obtain optimal dc perfor-

mance. A differential circuit may be used; a common heat

sink covering both sides (op amps and resistors) will help

reduce temperature effects.

Achieving good settling performance requires careful board

layout with multiple decoupling circuits and very clean

power and ground routing. It is important that digital switch-

ing currents do not flow across analog input (REFIN) and

output signals. Terminations must be broadband and near

the device. Measuring settling performance is quite chal-

lenging and requires several test systems to ensure settling

errors from the instruments are not included.

Dynamic performance characteristics (e.g., settling, rise

and fall times, etc.) were measured with the outputs termi-

nated to ground through 50

Ω resistors. SFDR was deter-

mined using a transformer to convert the output from differ-

ential to single-ended as shown in figure 5. The SPT5510 is

designed primarily for step and settle or narrowband RF

applications. The second harmonic generally dominates

wideband SFDR measurements, although close-in spurs

are very small.

BANDGAP VOLTAGE REFERENCE

The on-chip bandgap voltage reference is designed to bias

the non-inverting input of the reference amplifier (AMPINB)

through a resistor equal to RSET to help compensate the ref-

erence amplifier (see the following section). If the bandgap

voltage is required by another DAC, or elsewhere in the sys-

tem, it must be buffered with a precision op amp configured

as a high impedance (e.g., unity gain follower) buffer. A

resistor, or RC filter, plus a ferrite bead will help isolate the

output from the reference amplifier’s compensation and

high-frequency charge pulses produced during operation.

The output should always be very carefully checked for

oscillations using a sensitive, wideband oscilloscope and

spectrum analyzer.

REFERENCE AMPLIFIER

The reference amplifier is a highly temperature-stable driver

to bias the precision current sinks. The reference amplifier

should only be used to drive REFIN. Additional loads will

change the amplifier’s compensation, which can lead to

instability and other settling issues.

There are two reference amplifier outputs: AMPOUT and

AMPCC. AMPOUT has a 20 ohm series resistor between the

output of the reference amplifier and the AMPOUT pin;

AMPCC has a 10 ohm resistor. These parallel outputs aide

compensation and decoupling. The open-loop output

impedance is approximately 1200 ohms.

Reference amplifier compensation is key to achieving high

performance. Without proper compensation, oscillations

that affect accuracy and settling time will occur. Figure 6

shows a typical reference amplifier compensation circuit.

Note that several small value capacitors are used from

REFIN to ground. This is to provide suitably low impedance

IOUT

Figure 5 – Transformer Output Circuit

Figure 6 – Reference Amplifier Circuit

–

Ω

AMPCC

AMPOUT

Ω

VEE

20 pF

1 k

Ω

1 k

Ω

RSET

BGOUT

0.01

µF

Ref Amp

47 pF each

SPT5510 DAC

10 pF

0.01

µF

All components are ceramic chip-type.

AMPB

REFIN

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