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AD7541AKN 데이터시트(PDF) 4 Page - Analog Devices |
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AD7541AKN 데이터시트(HTML) 4 Page - Analog Devices |
4 / 8 page AD7541A –4– REV. B GENERAL CIRCUIT INFORMATION The simplified D/A circuit is shown in Figure 1. An inverted R-2R ladder structure is used—that is, the binarily weighted currents are switched between the OUT1 and OUT2 bus lines, thus maintaining a constant current in each ladder leg indepen- dent of the switch state. 10k Ω 10k Ω 10k Ω 20k Ω 20k Ω 20k Ω 20k Ω 20k Ω S1 S2 S3 S12 VREF OUT2 OUT1 RFEEDBACK BIT 12 (LSB) BIT 3 BIT 2 BIT 1 (MSB) DIGITAL INPUTS (DTL/TTL/CMOS COMPATIBLE) LOGIC: A SWITCH IS CLOSED TO IOUT1 FOR ITS DIGITAL INPUT IN A "HIGH" STATE. 10k Ω Figure 1. Functional Diagram (Inputs HIGH) The input resistance at VREF (Figure 1) is always equal to RLDR (RLDR is the R/2R ladder characteristic resistance and is equal to value “R”). Since RIN at the VREF pin is constant, the reference terminal can be driven by a reference voltage or a reference current, ac or dc, of positive or negative polarity. (If a current source is used, a low temperature coefficient external RFB is recommended to define scale factor.) EQUIVALENT CIRCUIT ANALYSIS The equivalent circuits for all digital inputs LOW and all digital inputs HIGH are shown in Figures 2 and 3. In Figure 2 with all digital inputs LOW, the reference current is switched to OUT2. The current source ILEAKAGE is composed of surface and junc- tion leakages to the substrate, while the I/4096 current source represents a constant 1-bit current drain through the termina- tion resistor on the R-2R ladder. The ON capacitance of the output N-channel switch is 200 pF, as shown on the OUT2 terminal. The OFF switch capacitance is 70 pF, as shown on the OUT1 terminal. Analysis of the circuit for all digital inputs HIGH, as shown in Figure 3 is similar to Figure 2; however, the ON switches are now on terminal OUT1, hence the 200 pF at that terminal. ILEAKAGE 70pF R ILEAKAGE 200pF I/4096 IREF R 15k Ω VREF RFB OUT1 OUT2 Figure 2. DAC Equivalent Circuit All Digital Inputs LOW ILEAKAGE 70pF R ILEAKAGE 200pF I/4096 IREF R 15k Ω VREF RFB OUT2 OUT1 Figure 3. DAC Equivalent Circuit All Digital Inputs HIGH APPLICATIONS UNIPOLAR BINARY OPERATION (2-QUADRANT MULTIPLICATION) Figure 4 shows the analog circuit connections required for uni- polar binary (2-quadrant multiplication) operation. With a dc reference voltage or current (positive or negative polarity) ap- plied at Pin 17, the circuit is a unipolar D/A converter. With an ac reference voltage or current, the circuit provides 2-quadrant multiplication (digitally controlled attenuation). The input/ output relationship is shown in Table II. R1 provides full-scale trim capability [i.e., load the DAC register to 1111 1111 1111, adjust R1 for VOUT = –VREF (4095/4096)]. Alternatively, Full Scale can be adjusted by omitting R1 and R2 and trimming the reference voltage magnitude. C1 phase compensation (10 pF to 25 pF) may be required for stability when using high speed amplifiers. (C1 is used to cancel the pole formed by the DAC internal feedback resistance and output capacitance at OUT1). Amplifier A1 should be selected or trimmed to provide VOS ≤ 10% of the voltage resolution at VOUT. Additionally, the ampli- fier should exhibit a bias current which is low over the tempera- ture range of interest (bias current causes output offset at VOUT equal to IB times the DAC feedback resistance, nominally 11 k Ω). The AD544L is a high speed implanted FET input op amp with low factory-trimmed VOS. 18 16 1 2 3 17 AD7541A VDD RFB VDD VREF PINS 4–15 DGND OUT1 OUT2 R1 * VIN BIT 1 – BIT 12 DIGITAL GROUND ANALOG COMMON R2 * C1 33pF AD544L (SEE TEXT) VOUT *REFER TO TABLE 1 Figure 4. Unipolar Binary Operation Table I. Recommended Trim Resistor Values vs. Grades Trim Resistor JN/AQ/SD KN/BQ/TD R1 100 Ω 100 Ω R2 47 Ω 33 Ω Table II. Unipolar Binary Code Table for Circuit of Figure 4 Binary Number in DAC MSB LSB Analog Output, VOUT 1 1 1 1 1 1 1 1 1 1 1 1 –VIN 4095 4096 1 0 0 0 0 0 0 0 0 0 0 0 –VIN 2048 4096 = –1/2 VIN 0 0 0 0 0 0 0 0 0 0 0 1 –VIN 1 4096 0 0 0 0 0 0 0 0 0 0 0 0 0 Volts |
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