AD557
and offset; therefore, no provisions have been made for such
user trims. If a small increase in scale is required, however, it
may be accomplished by slightly altering the effective gain of the
output buffer. A resistor in series with V
OUT
SENSE will
increase the output range. Note that decreasing the scale by put-
ting a resistor in series with GND will not work properly due to
the code-dependent currents in GND. Adjusting offset by
injecting dc at GND is not recommended for the same reason.
Figure 1. Functional Block Diagram
The high-speed output buffer amplifier is operated in the
noninverting mode with gain determined by the user-
connections at the output range select pin. The gain-setting
application resistors are thin film laser trimmed to match and
track the DAC resistors and to assure precise initial calibration
of the output range, 0 V to 2.56 V. The amplifier output stage is
an NPN transistor with passive pull-down for zero-based output
capability with a single power supply.
The internal precision voltage reference is of the patented
band-gap type. This design produces a reference voltage of
1.2 V and thus, unlike 6.3 V temperature-compensated Zeners,
may be operated from a single, low-voltage logic power supply.
The microprocessor interface logic consists of an 8-bit data latch
and control circuitry. Low power, small geometry and high
speed are advantages of the I
2
L design as applied to this section.
I
2
L is bipolar process compatible so that the performance of the
analog sections need not be compromised to provide on-chip
logic capabilities. The control logic allows the latches to be
operated from a decoded microprocessor address and write sig-
nal. If the application does not involve a
µP
or data bus, wiring
CS
and
CE
to ground renders the latches “transparent” for
direct DAC access.
Binary
0000 0000
0000 0001
0000 0010
0000 1111
0001 0000
0111 1111
1000 0000
1100 0000
1111 1111
Digital Input Code
Hexadecimal
00
01
02
0F
10
7F
80
C0
FF
Decimal
0
1
2
15
16
127
128
192
255
Output
Voltage
0
0.010 V
0.020 V
0.150 V
0.160 V
1.270 V
1.280 V
1.920 V
2.55 V
Figure 2. 0 V to 2.56 V Output Range
BIPOLAR –1.28 V TO +1.28 V OUTPUT RANGE
The AD557 was designed for operation from a single power
supply and is thus capable of providing only a unipolar 0 V to
+2.56 V output range. If a negative supply is available, bipolar
output ranges may be achieved by suitable output offsetting and
scaling. Figure 3 shows how a
±
1.28 V output range may be
achieved when a –5 V power supply is available. The offset is
provided by the AD589 precision 1.2 V reference which will
operate from a +5 V supply. The AD711 output amplifier can
provide the necessary
±
1.28 V output swing from
±
5 V supplies.
Coding is complementary offset binary.
Figure 3. Bipolar Operation of AD557 from
±
5 V Supplies
Applications
GROUNDING AND BYPASSING
CONNECTING THE AD557
The AD557 has been configured for low cost and ease of appli-
cation. All reference, output amplifier and logic connections are
made internally. In addition, all calibration trims are performed
at the factory assuring specified accuracy without user trims.
The only connection decision to be made by the user is whether
the output range desired is unipolar or bipolar. Clean circuit
board layout is facilitated by isolating all digital bit inputs on
one side of the package; analog outputs are on the opposite side.
UNIPOLAR 0 V TO +2.56 V OUTPUT RANGE
All precision converter products require careful application of
good grounding practices to maintain full rated performance.
Because the AD557 is intended for application in microcom-
puter systems where digital noise is prevalent, special care must
be taken to assure that its inherent precision is realized.
The AD557 has two ground (common) pins; this minimizes
ground drops and noise in the analog signal path. Figure 4
shows how the ground connections should be made.
It is often advisable to maintain separate analog and digital
grounds throughout a complete system, tying them common in
one place only. If the common tie-point is remote and accidental
disconnection of that one common tie-point occurs due to card
removal with power on, a large differential voltage between the
–3–
Figure 2 shows the configuration for the 0 V to +2.56 V full-
scale output range. Because of its precise factory calibration, the
AD557 is intended to be operated without user trims for gain
REV.A
AD [ ANALOG DEVICES ]
