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AD7628TE Scheda tecnica(PDF) 6 Page - Analog Devices |
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AD7628TE Scheda tecnica(HTML) 6 Page - Analog Devices |
6 / 8 page AD7628 –6– REV. A APPLICATIONS INFORMATION Application Hints To ensure system performance consistent with AD7628 specifi- cations, careful attention must be given to the following points: 1. GENERAL GROUND MANAGEMENT: AC or transient voltages between the AD7628 AGND and DGND can cause noise injection into the analog output. The simplest method of ensuring that voltages at AGND and DGND are equal is to tie AGND and DGND together at the AD7628. In more omplex systems where the AGND–DGND intertie is on the backplane, it is recommended that diodes be connected in inverse parallel between the AD7628 AGND and DGND pins (1N914 or equivalent). 2. OUTPUT AMPLIFIER OFFSET: CMOS DACs exhibit a code-dependent output resistance which, in turn, causes a code-dependent amplifier noise gain. The effect is a code- dependent differential nonlinearity term at the amplifier output that depends on VOS (VOS is amplifier input offset voltage). This differential nonlinearity term adds to the R/2R differential nonlinearity. To maintain monotonic operation, it is recommended that amplifier VOS be no greater than 10% of 1 LSB over the temperature range of interest. 3. HIGH FREQUENCY CONSIDERATIONS: The output capacitance of a CMOS DAC works in conjunction with the amplifier feedback resistance to add a pole to the open loop response. This can cause ringing or oscillation. Stability can be restored by adding a phase compensation capacitor in parallel with the feedback resistor. DYNAMIC PERFORMANCE The dynamic performance of the two DACs in the AD7628 will depend on the gain and phase characteristics of the output am- plifiers, together with the optimum choice of the PC board lay- out and decoupling components. Figure 6 shows the relationship between input frequency and channel-to-channel isolation. Figure 6. Channel-to-Channel Isolation Figure 7. Suggested PC Board Layout for AD7628 with AD644 Dual Op Amp Figure 7 shows a printed circuit layout for the AD7628 and the AD644 dual op amp, which minimizes feedthrough and crosstalk. SINGLE SUPPLY APPLICATIONS The AD7628 DAC R-2R ladder termination resistors are con- nected to AGND within the device. This arrangement is par- ticularly convenient for single supply operation because AGND may be biased at any voltage between DGND and VDD. Figure 8 shows a circuit that provides two +5 V to +8 V analog outputs by biasing AGND +5 V up from DGND. The two DAC refer- ence inputs are tied together and a reference input voltage is ob- tained without a buffer amplifier by making use of the constant and matched impedances of the DAC A and DAC B reference inputs. Current flows through the two DAC R-2R ladders into R1, and R1 is adjusted until the VREF A and VREF B inputs are at +2 V. The two analog output voltages range from +5 V to +8 V for DAC codes 00000000 to l l l l l l l l . Figure 8. AD7628 Single Supply Operation Figure 9 shows DAC A of the AD7628 connected in a positive reference, voltage switching mode. This configuration is useful because VOUT is the same polarity as VIN, allowing single supply operation. However, to retain specified linearity, VIN must be in the range 0 V to +2.5 V and the output buffered or loaded with a high impedance (see Figure 10). Note that the input voltage is connected to the DAC OUT A, and the output voltage is taken from the DAC VREF A pin. Figure 9. AD7628 Single Supply, Voltage Switching Mode Figure 10. Typical AD7628 Performance in Single Supply Voltage Switching Mode |
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