The feedback resistor should be matched to R
SET
to reduce
gain drift. Also, the op amp’s ground reference should be
the same as R
SET
’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 (REF
IN
) 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.
Figure 5 – Transformer Output Circuit
I
OUT
25
BANDGAP VOLTAGE REFERENCE
The on-chip bandgap voltage reference is designed to bias
the non-inverting input of the reference amplifier (AMP
INB
)
through a resistor equal to R
SET
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 REF
IN
. Additional loads will
change the amplifier’s compensation, which can lead to
instability and other settling issues.
There are two reference amplifier outputs: AMP
OUT
and
AMP
CC
. AMP
OUT
has a 20 ohm series resistor between the
output of the reference amplifier and the AMP
OUT
pin;
AMP
CC
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
REF
IN
to ground. This is to provide suitably low impedance
25
I
OUT
Figure 6 – Reference Amplifier Circuit
SPT5510 DAC
R
SET
1 kΩ
17
+
20
Ω
10
Ω
20
21
REF
IN
Ref Amp
AMP
OUT
19
50Ω
1 kΩ
0.01
µF
16
18
–
AMP
CC
10 pF
C1
C2
C3
BG
OUT
15
47 pF each
AMP
B
0.01
µF
V
EE
20 pF
All components are ceramic chip-type.
SPT5510
6
9/27/00
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
