AD8616 Supply Voltage

Question

Hi, 
 I'm still relatively inexperienced with circuit design and I am wondering if I am misunderstanding something fundamental about this particular op amp. The AD8615 series is a single supply op amp, but one of the listed application is photodiode preamplification. In general, one wants to recieve a signal from 0V to some voltage as the readout for the photodiode. SInce the headroom of the lower rail is something on the order of 10s of millivolts, you would want the lower rail to be below ground so that the signal can reach all the way down to 0V. This is what is shown in the sample circuit diagram of included in the datasheet. Where the supply voltage is +/- 2.5 volts. Now, when I read the absolute ratings, the input voltage is listed as gnd to Vs. and the supply voltage is 6V. So it seems like you cannot run this on dual supply voltages. The specifications of the op amp and the listed application does not seem to match. Can someone explain that?

FormerMember · Accepted Answer

Cheng, 
 You are not alone in your confusion. 
 When op amps came out in the 1960's and 1970's, they ran on +/-15V and usually had an NPN diffierential pair for the input stage 
 and an emitter follower output stage. See figures 2, 6, and 7 in MT-035: 
 http://www.analog.com/static/imported-files/tutorials/MT-035.pdf 
 Because the input differential pair will not work unless there is sufficient voltage across the base-emitter junction and the current 
 source, you usually had to stay away from the bottom power supply pin by one or two volts, sometimes more. Most of our 
 datasheets have a parameter in the spec table called "Input Voltage Range" (IVR). If both inputs are within this range, it 
 will act as an op amp. 
 For the output stage, you have the voltage required across the base driving circuit, plus the base-emitter voltage, so the output 
 cannot get to the top or bottom rail, usually one volt or more away. Again, our spec table will have output swing (Voh and Vol) 
 into a 2k load or a 10k load or xxx. 
 See MT-041: 
 http://www.analog.com/static/imported-files/tutorials/MT-041.pdf 
 As the digital guys tried to go faster, they turned to CMOS where all of the feature sizes were being reduced. This lead to some 
 processes that had breakdown voltages of 5.5V or 6V, so they clamored for 5V only op amps. So for op amps to run on ground 
 and 5V, we went to a PNP input stage, so the inputs could go to ground, but you couldn't take the inputs to V+. But by adding 
 a PNP diff pair and an NPN pair, the input stage would use one pair when it was close to one rail, and the other pair when the inputs 
 where close to the other rail. 
 For the output stage, we flipped the output transistors and instead of emitter followers, used common emitter or common source. 
 This still does not allow you to go exactly to the rails with the outputs, but certainly closer, say 100-200 mV. 
 So the official definition of a "single suppy op amp" is one where the input voltage range includes the V- pin (ground), and the 
 output gets close to the V- pin. (ground) 
 Let's say you have an op amp with a +/-15V spec table, and the IVR is +/-11V and the output swing is stated to be +/-12V into a 2k load. 
 See the 1981 op amp: OP-27: 
 http://www.analog.com/static/imported-files/data_sheets/OP27.pdf 
 There is no ground pin on an op amp, so the little electrons inside don't know where your ground is; they only know what is on the pins. 
 You could connect the OP27 on ground and plus 30V, and if you kept the inputs between +4V and +26V, the output would be 
 between +3V and +27V if you had a 2k resistor connected from the output to +15V . This would be single supply operation, 
 but the OP-27 is NOT a single supply op amp according to the official definition. 
 Some signal generators put out sine waves centered around ground, and spectrum analyzer inputs like to see sine waves 
 centered around ground, so we do test parts with +/- supplies. This is valid. 
 For figure 44 on the d.s., the photodiode is operated with reverse bias to reduce the capacitance so the system is faster. 
 So they ran it on +/-2.5V. The capacitance vs. reverse voltage characteristics are left as a homework assignment. (g) 
 Good luck on your studies..... 
 For a good reference on photodiodes and transimpedance amplifiers, see: 
 http://www.jensign.com/transimpedanceamp 
 Harry)

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AD8616 Supply Voltage