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AD8657 Datasheet(PDF) 3 Page - Analog Devices |
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AD8657 Datasheet(HTML) 3 Page - Analog Devices |
3 / 5 page Technical Article MS-2212 www.analog.com November 2011 | Page 3 of 5 ©2011 Analog Devices, Inc. All rights reserved. within the specified input voltage range (IVR) from the data sheet specification table. To measure supply current when open loop, such as operation as a comparator, see Figure 4 and Figure 5. Some low noise, bipolar op amps have diodes between the inputs to protect the differential input pair, so the maximum differential voltage is usually stated in the Absolute Maximum table as ±0.7 V. If there are internal series resistors, they are usually in the 500 Ω to 2 kΩ range. The Absolute Maximum table may state that the maximum differential voltage is ± supply voltage, but this does not mean that the part operates. A simplified internal schematic should be consulted. If one is not provided, a quick call to the manufacturer can resolve this. In these two configurations, the choice of resistor values is a little more critical. The resistor values should be low enough to cause the differential input voltage to be at least 0.5 V to guarantee that the output is driven hard into the rail but high enough not to damage the internal diodes. Values were chosen to limit the input current to less than 1 mA. Figure 4. Comparator, output low Figure 5. Comparator, output high Table 1 lists the maximum supply current specification from the data sheets, the measured supply current with the op amp connected as a follower with VIN halfway between the supply pins (Figure 3), and the supply current with the output forced low (Figure 4) and forced high (Figure 5). Classic op amp and comparator Table 1 shows that the classic LM358 and LM393 are well behaved, as expected. Bipolar Rail-to-Rail Op Amps All the bipolar rail-to-rail output op amps have supply current greater than the “maximum” op amp supply current in one or both comparator circuits. There are several ways to drive the output stage, so some methods will result in a supply current increase when driving to one rail or the other. Without being privy a manufacturer’s internal schematics, one cannot comment on the behavior. For the OP284, the second stage and output stage simplified schematic is shown on the data sheet. See Figure 6. If VOUT is driven high by Q5/Q3/Q4, the supply current will be a function of the values of R4 and R6. These values are selected to maximize the op amp performance and minimize die area, not comparator operation. When VOUT is driven low by Q6/R1/Q1, the supply current will be determined by R1. Again, the values of R1, I1, etc. are chosen for op amp performance, not comparator performance. |
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