Datasheet AD557 (Analog Devices) - 4

AD557 CIRCUIT DESCRIPTION CONNECTING THE AD557
The AD557 consists of four major functional blocks fabricated The AD557 has been configured for low cost and ease of appli- on a single monolithic chip (see Figure 1). The main D/A con- cation. All reference, output amplifier and logic connections are verter section uses eight equally weighted laser-trimmed current made internally. In addition, all calibration trims are performed sources switched into a silicon-chromium thin-film R/2R resistor at the factory assuring specified accuracy without user trims. ladder network to give a direct but unbuffered 0 mV to 400 mV The only connection decision to be made by the user is whether output range. The transistors that form the DAC switches are the output range desired is unipolar or bipolar. Clean circuit PNPs; this allows direct positive-voltage logic interface and a board layout is facilitated by isolating all digital bit inputs on zero-based output range. one side of the package; analog outputs are on the opposite side.
CONTROL DIGITAL INPUT DATA (BUS) UNIPOLAR 0 V TO 2.56 V OUTPUT RANGE INPUTS BIT1 BIT8
Figure 2 shows the configuration for the 0 V to 2.56 V full-
(MSB) CS CE (LSB) +VCC GND GND
scale output range. Because of its precise factory calibration, the AD557 is intended to be operated without user trims for gain
l2L CONTROL l2L LATCHES
and offset; therefore, no provisions have been made for such user
LOGIC
trims. If a small increase in scale is required, however, it may be
OUTPUT BAND-GAP
accomplished by slightly altering the effective gain of the output
8-BIT VOLTAGE-SWITCHING AMP REFERENCE D-TO-A CONVERTER
buffer. A resistor in series with VOUT SENSE will increase the
VOUT CONTROL
output range. Note that decreasing the scale by putting a resistor
AMP V
in series with GND will not work properly due to the code-
OUT SENSE A
dependent currents in GND. Adjusting offset by injecting dc at
VOUT SENSE B
GND is not recommended for the same reason. Figure 1. Functional Block Diagram
OUTPUT AMP
The high-speed output buffer amplifier is operated in the nonin-
16 VOUT
verting mode with gain determined by the user-connections at the output range select pin. The gain-setting application
15 VOUT SENSE A
resistors are thin film laser trimmed to match and track the
14 VOUT SENSE B
DAC resistors and to assure precise initial calibration of the out-
13 GND
put 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. Figure 2. 0 V to 2.56 V Output Range The internal precision voltage reference is of the patented band-gap type. This design produces a reference voltage of
BIPOLAR –1.28 V TO +1.28 V OUTPUT RANGE
1.2 V and thus, unlike 6.3 V temperature-compensated Zeners, The AD557 was designed for operation from a single power may be operated from a single, low-voltage logic power supply. supply and is thus capable of providing only a unipolar 0 V to The microprocessor interface logic consists of an 8-bit data 2.56 V output range. If a negative supply is available, bipolar latch and control circuitry. Low power, small geometry and high output ranges may be achieved by suitable output offsetting and speed are advantages of the I2L design as applied to this section. scaling. Figure 3 shows how a ± 1.28 V output range may be I2L is bipolar process compatible so that the performance of the achieved when a –5 V power supply is available. The offset is analog sections need not be compromised to provide on-chip provided by the AD589 precision 1.2 V reference which will logic capabilities. The control logic allows the latches to be operate from a 5 V supply. The AD711 output amplifier can operated from a decoded microprocessor address and write sig- provide the necessary ± 1.28 V output swing from ± 5 V supplies. nal. If the application does not involve a Coding is complementary offset binary. µP or data bus, wiring CS and CE to ground renders the latches “transparent” for
5k
⍀ direct DAC access.
VOUT = 0V TO 2.56V +5V Digital Input Code Output AD711 0.01
␮
F Binary Hexadecimal Decimal Voltage 5k
⍀
AD557 4.53k
0000 0000 00 0 0 ⍀
VO 500
⍀
0.01
␮
F +1.28 TO
0000 0001 01 1 0.010 V
BIPOLAR –1.27 OFFSET 1.5k
⍀ 0000 0010 02 2 0.020 V
ADJUST AD589 VIN
0000 1111 0F 15 0.150 V
–5V –1.2V
0001 0000 10 16 0.160 V
V 0.01
␮
F 4.7k
⍀
INPUT CODE OUT 0 0 0 0 0 0 0 0 +1.28V
0111 1111 7F 127 1.270 V
1 0 0 0 0 0 0 0 0V 1 1 1 1 1 1 1 1
1000 0000 80 128 1.280 V
–1.27V –5V
1100 0000 C0 192 1.920 V 1111 1111 FF 255 2.55 V Figure 3. Bipolar Operation of AD557 from ±5 V Supplies REV. B –3–