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AD5415 Datasheet(PDF) 17 Page - Analog Devices |
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AD5415 Datasheet(HTML) 17 Page - Analog Devices |
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17 / 28 page  AD5415 Rev. 0 | Page 17 of 28 SINGLE-SUPPLY APPLICATIONS VOLTAGE SWITCHING MODE OF OPERATION Figure 35 shows these DACs operating in the voltage switching mode. The reference voltage, VIN, is applied to the IOUT1 pin, IOUT2 is connected to AGND, and the output voltage is available at the VREF terminal. In this configuration, a positive reference voltage results in a positive output voltage, making single- supply operation possible. The output from the DAC is voltage at a constant impedance (the DAC ladder resistance). Therefore, an op amp is necessary to buffer the output voltage. The reference input no longer sees a constant input impedance, but one that varies with code. So, the voltage input should be driven from a low impedance source. VOUT VDD GND VIN IOUT2 IOUT1 RFB VDD VREF R2 R1 NOTES 1. SIMILAR CONFIGURATION FOR DACB 2. C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED IF A1 IS A HIGH SPEED AMPLIFIER. Figure 35. Single-Supply Voltage Switching Mode Note that VIN is limited to low voltages, because the switches in the DAC ladder no longer have the same source-drain drive voltage. As a result, their on resistance differs and this degrades the integral linearity of the DAC. Also, VIN must not go negative by more than 0.3 V or an internal diode is turned on, exceeding the maximum ratings of the device. In this type of application, the full range of multiplying capability of the DAC is lost. POSITIVE OUTPUT VOLTAGE The output voltage polarity is opposite to the VREF polarity for dc reference voltages. To achieve a positive voltage output, an applied negative reference to the input of the DAC is preferred over the output inversion through an inverting amplifier because of the resistors’ tolerance errors. To generate a negative reference, the reference can be level-shifted by an op amp such that the VOUT and GND pins of the reference become the virtual ground and −2.5 V, respectively, as shown in Figure 36. VDD RFB IOUT1 IOUT2 C1 VOUT = 0 TO +2.5V GND VDD = 5V VREF NOTES: 1ADDITIONAL PINS OMITTED FOR CLARITY. 2C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED, IF A1 IS A HIGH SPEED AMPLIFIER. 1/2 AD8552 12-BIT DAC 1/2 AD8552 ADR03 VOUT VIN GND –5V +5V –2.5V Figure 36. Positive Voltage Output with Minimum of Components ADDING GAIN In applications where the output voltage is required to be greater than VIN, gain can be added with an additional external amplifier, or it can also be achieved in a single stage. It is important to take into consideration the effect of temperature coefficients of the thin film resistors of the DAC. Simply placing a resistor in series with the RFB resistor causes mismatches in the temperature coefficients, resulting in larger gain temperature coefficient errors. Instead, the circuit in Figure 37 is a recom- mended method of increasing the gain of the circuit. R1, R2, and R3 should all have similar temperature coefficients, but they need not match the temperature coefficients of the DAC. This approach is recommended in circuits where gains of greater than 1 are required. VDD RFB IOUT1 IOUT2 C1 GND VDD VREF NOTES: 1ADDITIONAL PINS OMITTED FOR CLARITY. 2C1 PHASE COMPENSATION (1pF TO 2pF) MAY BE REQUIRED, IF A1 IS A HIGH SPEED AMPLIFIER. 12-BIT DAC VIN R2 R3 R2 VOUT R1 = R2R3 R2 + R3 GAIN = R2 + R3 R2 Figure 37. Increasing the Gain of the Current Output DAC DIVIDER OR PROGRAMMABLE GAIN ELEMENT Current-steering DACs are very flexible and lend themselves to many different applications. If this type of DAC is connected as the feedback element of an op amp and RFB is used as the input resistor, as shown in Figure 38, then the output voltage is inversely proportional to the digital input fraction, D. For D equal to 1 − 2n, the output voltage is VOUT = −VIN/D = −VIN/(1 −2−n) |
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