Datasheet AD648 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Dual Precision, Low Power BiFET Op Amp |
| Pages / Page | 12 / 8 — AD648. D/A CONVERTER BIPOLAR OUTPUT BUFFER. SIGN BIT. BINARY NUMBER IN … |
| Revision | E |
| File Format / Size | PDF / 209 Kb |
| Document Language | English |
AD648. D/A CONVERTER BIPOLAR OUTPUT BUFFER. SIGN BIT. BINARY NUMBER IN DAC REGISTER ANALOG OUTPUT. 1111 1111 1111. +VIN
Model Line for this Datasheet
Text Version of Document
AD648
Figure 23c shows a diode clamp protection scheme for an I-to-V CMOS DAC’s output current to a voltage and provides the converter using low leakage diodes. Because the diodes are necessary level shifting to achieve a bipolar voltage output. The connected to the op amp’s summing junction, which is a virtual circuit operates with a 12-bit plus sign input code. The transfer ground, their leakage contribution is minimal. function is shown in Figure 25. The AD7592 is a fully protected dual CMOS SPDT switch with data latches. R4 and R5 should match to within 0.01% to main- tain the accuracy of the converter. A mismatch between R4 and R5 introduces a gain error. Overall gain is trimmed by adjusting RIN. The AD648’s low input offset voltage, low drift over tem- perature, and excellent dynamics make it an attractive low power output buffer. Figure 23c. I-to-V Converter with Diode Input Protection The input offset voltage of the AD648 output amplifier results in an output error voltage. This error voltage equals the input Exceeding the negative common-mode range on either input offset voltage of the op amp times the noise gain of the amplifier. terminal causes a phase reversal at the output, forcing the ampli- fier output to the corresponding high or low state. Exceeding That is: the negative common mode on both inputs simultaneously forces the output high. Exceeding the positive common-mode VOS Output = VOS Input 1 + RFB R range on a single input does not cause a phase reversal; but if O both inputs exceed the limit, the output will be forced high. In RFB is the feedback resistor for the op amp, which is internal to all cases, normal amplifier operation is resumed when input the DAC. RO is the DAC’s R-2R ladder output resistance. The voltages are brought back within the common-mode range. value of RO is code dependent. This has the effect of changing the offset error voltage at the amplifier’s output. An output
D/A CONVERTER BIPOLAR OUTPUT BUFFER
amplifier with a sub millivolt input offset voltage is needed to The circuit in Figure 24 provides 4 quadrant multiplication with preserve the linearity of the DAC’s transfer function. a resolution of 12 bits. The AD648 is used to convert the AD7545 Figure 24. 12-Bit Plus Sign Magnitude D/A Converter
SIGN BIT BINARY NUMBER IN DAC REGISTER ANALOG OUTPUT 0 1111 1111 1111 +VIN
ⴛ
(4095/4096) 0 0000 0000 0000 0 V 1 0000 0000 0000 0 V 1 1111 1111 1111 –VIN
ⴛ
(4095/4096) NOTE SIGN BIT AT “0“ CONNECTS THE NONINVERTING INPUT OF A2 TO ANALOG COMMON
Figure 25. Sign Magnitude Code Table –8– REV. E
Figure 23c shows a diode clamp protection scheme for an I-to-V CMOS DAC’s output current to a voltage and provides the converter using low leakage diodes. Because the diodes are necessary level shifting to achieve a bipolar voltage output. The connected to the op amp’s summing junction, which is a virtual circuit operates with a 12-bit plus sign input code. The transfer ground, their leakage contribution is minimal. function is shown in Figure 25. The AD7592 is a fully protected dual CMOS SPDT switch with data latches. R4 and R5 should match to within 0.01% to main- tain the accuracy of the converter. A mismatch between R4 and R5 introduces a gain error. Overall gain is trimmed by adjusting RIN. The AD648’s low input offset voltage, low drift over tem- perature, and excellent dynamics make it an attractive low power output buffer. Figure 23c. I-to-V Converter with Diode Input Protection The input offset voltage of the AD648 output amplifier results in an output error voltage. This error voltage equals the input Exceeding the negative common-mode range on either input offset voltage of the op amp times the noise gain of the amplifier. terminal causes a phase reversal at the output, forcing the ampli- fier output to the corresponding high or low state. Exceeding That is: the negative common mode on both inputs simultaneously forces the output high. Exceeding the positive common-mode VOS Output = VOS Input 1 + RFB R range on a single input does not cause a phase reversal; but if O both inputs exceed the limit, the output will be forced high. In RFB is the feedback resistor for the op amp, which is internal to all cases, normal amplifier operation is resumed when input the DAC. RO is the DAC’s R-2R ladder output resistance. The voltages are brought back within the common-mode range. value of RO is code dependent. This has the effect of changing the offset error voltage at the amplifier’s output. An output
D/A CONVERTER BIPOLAR OUTPUT BUFFER
amplifier with a sub millivolt input offset voltage is needed to The circuit in Figure 24 provides 4 quadrant multiplication with preserve the linearity of the DAC’s transfer function. a resolution of 12 bits. The AD648 is used to convert the AD7545 Figure 24. 12-Bit Plus Sign Magnitude D/A Converter
SIGN BIT BINARY NUMBER IN DAC REGISTER ANALOG OUTPUT 0 1111 1111 1111 +VIN
ⴛ
(4095/4096) 0 0000 0000 0000 0 V 1 0000 0000 0000 0 V 1 1111 1111 1111 –VIN
ⴛ
(4095/4096) NOTE SIGN BIT AT “0“ CONNECTS THE NONINVERTING INPUT OF A2 TO ANALOG COMMON
Figure 25. Sign Magnitude Code Table –8– REV. E
