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AD7538TQ Datasheet(PDF) 7 Page - Analog Devices |
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AD7538TQ Datasheet(HTML) 7 Page - Analog Devices |
7 / 8 page AD7538 REV. A –7– The transfer function of Figure 7 is: Gain = VOUT V IN = – REQ RFB (1) REQ is the equivalent transfer impedance of the DAC from the VREF pin to the IOUT pin and can be expressed as REQ = 2n RIN N (2) Where: n is the resolution of the DAC Where: N is the DAC input code in decimal Where: RIN is the constant input impedance Where: of the DAC (RIN = RLAD) Substituting this expression into Equation 1 and assuming zero gain error for the DAC (RIN = RFB) the transfer function simpli- fies to VOUT VIN = – 2n N (3) The ratio N/2 n is commonly represented by the term D and, as such, is the fractional representation of the digital input word. VOUT VIN = – –2n N = –1 D (4) Equation 4 indicates that the gain of the circuit can be varied from 16,384 down to unity (actually 16,384/16,383) in 16,383 steps. The all 0s code is never applied. This avoids an open- loop condition thereby saturating the amplifier. With the all 0s code excluded there remains 2 n – 1 possible input codes allow- ing a choice of 2 n – 1 output levels. In dB terms the dynamic range is 20 log10 VOUT VIN = 20 log 10 (2 n –1) = 84 dB. APPLICATION HINTS Output Offset: CMOS D/A converters in circuits such as Fig- ures 4 and 5 exhibit a code dependent output resistance which in turn can cause a code dependent error voltage at the output of the amplifier. The maximum amplitude of this error, which adds to the D/A converter nonlinearity, depends on VOS, where VOS is the amplifier input offset voltage. To maintain specified accuracy with VREF at 10 V, it is recommended that VOS be no greater than 0.25 mV, or (25 × 10–6) (V REF), over the tempera- ture range of operation. The AD711 is a suitable op amp. The op amp has a wide bandwidth and high slew rate and is recom- mended for ac and other applications requiring fast settling. General Ground Management: Since the AD7538 is speci- fied for high accuracy, it is important to use a proper grounding technique. AC or transient voltages between 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 AD7538. In more complex systems where the AGND and DGND intertie is on the backplane, it is recommended that two diodes be connected in inverse parallel between the AD7538 AGND and DGND pins (1N914 or equivalent). MICROPROCESSOR INTERFACING The AD7538 is designed for easy interfacing to 16-bit micro- processors and can be treated as a memory mapped peripheral. This reduces the amount of external logic needed for interfacing to a minimal. AD7538-8086 INTERFACE Figure 8 shows the 8086 processor interface to a single device. In this setup the double buffering feature (using LDAC) of the DAC is not used. The 14-bit word is written to the DAC in one MOV instruction and the analog output responds immediately. Figure 8. AD7538-8086 Interface Circuit In a multiple DAC system the double buffering of the AD7538 allows the user to simultaneously update all DACs. In Figure 9, a 14-bit word is loaded to the Input Registers of each of the DACs in sequence. Then, with one instruction to the appropri- ate address, CS4 (i.e., LDAC) is brought low, updating all the DACs simultaneously. Figure 9. AD7538-8086 Interface: Multiple DAC System |
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