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AD7541AKN Datasheet(PDF) 6 Page - Analog Devices |
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AD7541AKN Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page AD7541A –6– REV. B APPLICATIONS HINTS Output Offset: CMOS D/A converters exhibit a code-dependent output resistance which in turn can cause a code-dependent error voltage at the output of the amplifier. The maximum am- plitude of this offset, which adds to the D/A converter nonlin- earity, is 0.67 VOS where VOS is the amplifier input offset voltage. To maintain monotonic operation it is recommended that VOS be no greater than (25 × 10–6) (V REF) over the tempera- ture range of operation. Suitable op amps are AD517L and AD544L. The AD517L is best suited for fixed reference appli- cations with low bandwidth requirements: it has extremely low offset (50 µV) and in most applications will not require an offset trim. The AD544L has a much wider bandwidth and higher slew rate and is recommended for multiplying and other appli- cations requiring fast settling. An offset trim on the AD544L may be necessary in some circuits. Digital Glitches: One cause of digital glitches is capacitive coupling from the digital lines to the OUT1 and OUT2 termi- nals. This should be minimized by screening the analog pins of the AD7541A (Pins 1, 2, 17, 18) from the digital pins by a ground track run between Pins 2 and 3 and between Pins 16 and 17 of the AD7541A. Note how the analog pins are at one end of the package and separated from the digital pins by VDD and GND to aid screening at the board level. On-chip capacitive coupling can also give rise to crosstalk from the digital-to-analog sections of the AD7541A, particularly in circuits with high cur- rents and fast rise and fall times. Temperature Coefficients: The gain temperature coefficient of the AD7541A has a maximum value of 5 ppm/ °C and a typi- cal value of 2 ppm/ °C. This corresponds to worst case gain shifts of 2 LSBs and 0.8 LSBs, respectively, over a 100 °C temperature range. When trim resistors R1 and R2 are used to adjust full- scale range, the temperature coefficient of R1 and R2 should also be taken into account. The reader is referred to Analog Devices Application Note “Gain Error and Gain Temperature Coefficient of CMOS Multiplying DACs,” Publication Number E630c-5-3/86. SINGLE SUPPLY OPERATION Figure 7 shows the AD7541A connected in a voltage switching mode. OUT1 is connected to the reference voltage and OUT2 is connected to GND. The D/A converter output voltage is available at the VREF pin (Pin 17) and has a constant output impedance equal to RLDR. The feedback resistor RFB is not used in this circuit. 1 2 PINS 4–15 AD7541A RFB VREF GND OUT1 OUT2 BIT 1 – BIT 12 16 18 17 3 NOT USED VDD VOUT = 0V TO +10V R2 30k Ω R1 10k Ω SYSTEM GROUND V+ V– CA3140B VDD = +15V VREF +2.5V VOUT ±VREF D (1 +R2/R1) WHERE 0 ≤ D ≤ 1 i.e., D IS A FRACTIONAL REPRESENTATION OF THE DIGITAL INPUT 15 4 Figure 7. Single Supply Operation Using Voltage Switch- ing Mode The reference voltage must always be positive. If OUT1 goes more than 0.3 V less than GND, an internal diode will be turned on and a heavy current may flow causing device damage (the AD7541A is, however, protected from the SCR latch-up phenomenon prevalent in many CMOS devices). Suitable refer- ences include the AD580 and AD584. The loading on the reference voltage source is code-dependent and the response time of the circuit is often determined by the behavior of the reference voltage with changing load conditions. To maintain linearity, the voltage at OUT1 should remain within 2.5 V of GND, for a VDD of 15 V. If VDD is reduced from 15 V or the reference voltage at OUT1 increased to more than 2.5 V, the differential nonlinearity of the DAC will increase and the linearity of the DAC will be degraded. SUPPLEMENTAL APPLICATION MATERIAL For further information on CMOS multiplying D/A converters, the reader is referred to the following texts: CMOS DAC Application Guide, Publication Number G872b-8-1/89 available from Analog Devices. Gain Error and Gain Temperature Coefficient of CMOS Multiplying DACs Application Note, Publication Number E630c-5-3/86 available from Analog Devices. Analog-Digital Conversion Handbook—available from Analog Devices. |
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