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AD7111ABN Folha de dados(PDF) 6 Page - Analog Devices |
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AD7111ABN Folha de dados(HTML) 6 Page - Analog Devices |
6 / 8 page AD7111/AD7111A REV. 0 –6– For example, the AD7111L is guaranteed monotonic in 0.375 dB steps from 0 dB to –54 dB inclusive and in 0.75 dB steps from 0 dB to –72 dB inclusive. To achieve monotonic operation over the entire 88.5 dB range it is necessary to select input codes so that the attenuation step size at any point is consistent with the step size guaranteed for monotonic operation at that point. EQUIVALENT CIRCUIT ANALYSIS Figure 2 shows a simplified circuit of the D/A converter section of the AD7111/AD7111A, and Figure 3 gives an approximate equivalent circuit. The current source ILEAKAGE is composed of surface and junc- tion leakages. The resistor R0 as shown in Figure 3 is the equivalent output resistance of the device which varies with in- put code (excluding all 0s code) from 0.8R to 2R. R is typically 12 k Ω. C OUT is the capacitance due to the N channel switches and varies from about 20 pF to 50 pF depending upon the digi- tal input. For further information on CMOS multiplying D/A converters, refer to “CMOS DAC Application Guide” which is available from Analog Devices, Publication Number G872b–8–1/89. Figure 2. Simplified D/A Circuit of AD7111/AD7111A Figure 3. Equivalent Analog Output Circuit of AD7111/AD7111A DYNAMIC PERFORMANCE The dynamic performance of the AD7111/AD7111A will depend upon the gain and phase characteristics of the output amplifier, together with the optimum choice of PC board layout and decoupling components. Circuit layout is most important if the optimum performance of the AD7111/AD7111A is to be achieved. Most application problems stem from either poor lay- out, grounding errors, or inappropriate choice of amplifier. It is recommended that when using thc AD7111/AD7111A with a high speed amplifier, a capacitor (C1) he connected in the feedback path as shown in Figure 1. This capacitor, which should be between 10 pF and 30 pF, compensates for the phase lag introduced by the output capacitance of the D/A converter. Figures 4 and 5 show the performance of the AD7111/AD7111A using the AD711, a high speed, low cost BiFET amplifier, and the OP275, a dual, bipolar/JFET, audio amplifier. The perfor- mance without C1 is shown in the middle trace and the re- sponse with C1 in circuit shown in the bottom trace. 10 90 100 0% A1 –0.2V 200ns 5V 5V V OUT V OUT C1 = 0pF C1 = 15pF DATA CHANGE MSB DATA CHANGE FROM 80H TO 00H. Figure 4. Response of AD7111/AD7111A with AD711 10 90 100 0% A1 200ns 5V 5V V OUT V OUT C1 = 0pF C1 = 15pF DATA CHANGE MSB DATA CHANGE FROM 80H TO 00H. 0.4V Figure 5. Response of AD7111/AD7111A with 1/2 OP275 In conventional CMOS D/A converter design, parasitic capaci- tance in N-channel D/A converter switches can give rise to glitches on the D/A converter output. These glitches result from digital feedthrough. The AD7111/AD7111A has been designed to minimize these glitches as much as possible. For operation beyond 250 kHz, capacitor C1 may be reduced in value. This gives an increase in bandwidth at the expense of a poorer transient response as shown in Figures 5 and 11. In cir- cuits where C1 is not included, the high frequency roll-off point is primarily determined by the characteristics of the output am- plifier and not the AD7111/AD7111A. Feedthrough and absolute accuracy are sensitive to output leak- age current effects. For this reason it is recommended that the operating temperature of the AD7111/AD7111A be kept as close to 25 °C as is practically possible, particularly where the device’s performance at high attenuation levels is important. A typi- cal plot of leakage current vs. temperature is shown in Figure 10. Some solder fluxes and cleaning materials can form slightly con- ductive films which cause leakage effects between analog input and output. The user is cautioned to ensure that the manufacturing process for circuits using thc AD7111/AD7111A does not allow such films to form. Otherwise the feedthrough, accuracy and maximum usable range will be affected. STATIC ACCURACY PERFORMANCE The D/A converter section of the AD7111/AD7111A consists of a 17-bit R-2R type converter. To obtain optimum static perfor- mance at this level of resolution it is necessary to pay great attention to amplifier selection, circuit grounding, etc. Amplifier input has current results in a dc offset at the output of the amplifier due to the current flowing through the feedback resistor RFB. It is recommended that an amplifier with an input bias current of less than 10 nA be used (e.g., AD711) to mini- mize this offset. |
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