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AD7545 Fiches technique(PDF) 5 Page - Intersil Corporation |
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AD7545 Fiches technique(HTML) 5 Page - Intersil Corporation |
5 / 7 page 10-14 bus systems, data on the data bus is not always valid for the whole period during which WR is low and as a result invalid data can briefly occur at the D/A converter inputs during a write cycle. Such invalid data can cause unwanted glitches at the output of the D/A converter. The solution to this prob- lem, if it occurs, is to retime the write pulse (WR) so that it only occurs when data is valid. Another cause of digital glitches is capacitive coupling from the digital lines to the OUT1 and AGND terminals. This should be minimized by isolating the analog pins of the AD7545 (pins 1, 2, 19, 20) from the digital pins by a ground track run between pins 2 and 3 and between pins 18 and 19 of the AD7545. Note how the analog pins are at one end of the package and separated from the digital pins by VDD and DGND to aid isolation at the board level. On-chip capacitive coupling can also give rise to crosstalk from the digital to analog sections of the AD7545, particularly in circuits with high currents and fast rise and fall times. This type of crosstalk is minimized by using VDD = +5V. However, great care should be taken to ensure that the +5V used to power the AD7545 is free from digitally induces noise. Temperature Coefficients The gain temperature coefficient of the AD7545 has a maxi- mum value of 5ppm/oC and a typical value of 2ppm/oC. This corresponds to worst case gain shifts of 2 LSBs and 0.8 LSBs respectively over a 100oC 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. Basic Applications Figures 5 and 6 show simple unipolar and bipolar circuits using the AD7545. Resistor R1 is used to trim for full scale. Capacitor C1 provides phase compensation and helps pre- vent overshoot and ringing when using high speed op amps. Note that the circuits of Figures 5 and 6 have constant input impedance at the VREF terminal. The circuit of Figure 4 can either be used as a fixed reference D/A converter so that it provides an analog output voltage in the range 0V to -VIN (note the inversion introduced by the op amp) or VIN can be an AC signal in which case the circuit behaves as an attenuator (2-Quadrant Multiplier). VIN can be any voltage in the range -20V ≤ VIN ≤ +20V (provided the op amp can handle such voltages) since VREF is permitted to exceed VDD. Table 2 shows the code relationship for the circuit of Figure 4. Figure 5 and Table 3 illustrate the recommended circuit and code relationship for bipolar operation. The D/A function itself uses offset binary code and inverter U1 on the MSB line con- verts 2’s complement input code to offset binary code. If appro- priate, inversion of the MSB may be done in software using an exclusive -OR instruction and the inverter omitted. R3, R4 and R5 must be selected to match within 0.01% and they should be the same type of resistor (preferably wire-wound or metal foil), so that their temperature coefficients match. Mismatch of R3 value to R4 causes both offset and full scale error. Mismatch of R5 to R4 and R3 causes full scale error. The choice of the operational amplifiers in Figure 4 and Figure 5 depends on the application and the trade off between required precision and speed. Below is a list of operational amplifiers which are good candidates for many applications. The main selection criteria for these operational amplifiers is to have low VOS, low VOS drift, low bias current and low settling time. These amplifiers need to maintain the low nonlinearity and monotonic operation of the D/A while providing enough speed for maximum converter performance. Operational Amplifiers HA5127 Ultra Low Noise, Precision HA5137 Ultra Low Noise, Precision, Wide Band HA5147 Ultra Low Noise, Precision, High Slew Rate HA5170 Precision, JFET Input TABLE 1. RECOMMENDED TRIM RESISTOR VALUES vs GRADES FOR VDD = +5V TRIM RESISTOR J, A, S K, B R1 500 Ω 200 Ω R2 150 Ω 68 Ω TABLE 2. UNIPOLAR BINARY CODE TABLE FOR CIRCUIT OF FIGURE 4 BINARY NUMBER IN DAC REGISTER ANALOG OUTPUT 1111 1111 1111 1000 0000 0000 0000 0000 0001 0000 0000 0000 0V TABLE 3. 2’S COMPLEMENT CODE TABLE FOR CIRCUIT OF FIGURE 5 DATA INPUT ANALOG OUTPUT 0111 1111 1111 0000 0000 0001 0000 0000 0000 0V 1111 1111 1111 1000 0000 0000 V IN 4095 4096 ------------- – V IN 2048 4096 ------------- – 1 2 --- V IN – = V IN 1 4096 ------------- – +V IN 2047 2048 ------------- • +V IN 1 2048 ------------- • V IN 1 2048 ------------- • – V IN 2048 2048 ------------- • – AD7545 |
Numéro de pièce similaire - AD7545 |
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Description similaire - AD7545 |
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