ad8310: Level shifting the output

Question I have the same circuit like the AD8310 evaluation board. I need 150mV at the pin 4 (out) max when I close the RF In. I got 0.4V when I close the RF In. Can you please send to me solution/application for offset? I tried to work with the OFLT pin the best results (0.4V_was at 1.9V. Answer The output voltage of the AD8310 is approximately 0.4V to 2.6V which is simply defined by the headroom of the output stage. A simple op amp circuit connected to the AD8310 output can apply a gain and offset correction to this output signal so that the signal levels for mimimum and maximum received signal level correspond to the requirements of the following stage. You can use the circuit shown in the data converter handbook http://www.analog.com/library/analogDialogue/archives/39-06/data_conversion_hand book.html Chapter 6 page 6.11 figure 6.10A. If you don't have the appropriate low impedance Vref available, you can replace R1 with a resistive divider (R1a and R1b) between the supply and ground. R1a goes from the inverting input to Vsupply and R1b goes from the intverting input to ground. (R1a x R1b)/(R1a+R1b) = R1 (for the calculations of signal gain in figure 6.11 to remian true.) Vref = Vsupply x R1b/(R1a + R1b) The OFLT pin is for the on-chip high pass filter bandwidth setting, and cannot be used to level shift the AD8310 output.

AD8310 optical input power

Question Some times ago we have discussed about an application for measuring the optical input power with the AD8307AR. I have got some samples and I have built a circuitry which is attached at the end of this document. Could you or an responsible application engineer / chip designer check if this would work. At least I have got the impression that it is not easy to get accurate results. Furthermore I have also attached our measurement results. So you can easily see that the output voltage is not a linear function over the optical input power. I have theoretically checked the functionallity and this should give a linear output level in the range of 0.7V up to 1.7V. What are the main features of our application and of the circuitry? I will start with the main features: - first of all we have to measure a dc voltage. This is now a fact. The last times I was not sure what is to measure (DC bias or the overlayed AC voltage). - the above mentioned fact means a dc-coupled application for the AD8307AR. (You can see it in may application circuitry). - one end of the measuring resistor is connected to +3.3V. We have noticed that there is a big influence of the absolute voltage to the output level if there is no optical input power (then I expect only a noise level which will result in a finite output voltage nearby 0.5V or lower) . Unfortunately this basis output level is in the near of 1V and for a measuring resistor of 1k instead of 180 ohms it is nearly 1.5V. I guessed that there is some ripple on the power supply voltage (+3.3V). However I couldn't see anything with the scope. Here are my questions: - Would you prefer the AD8310AR (has an additional output buffer and better datas than AD8307AR) or another device instead of the AD8307AR? - Do you see any possibility to reduce the big influence of the supply voltage (+3.3V)? I will try ones more to feed the circuitry separately.- Is it necessary to shield the whole circuitry? If yes, this would raise the effort quite much.- The DC-voltages at the differential inputs is about +3V whereby the supply voltage for the AD8307AR is +5V. Does this work properly? In the datasheet is only mentioned that the DC level has to be min. 2V over GROUND. - Shall I provide a DC voltage at the OFS input or is it ok. like I have done it? Thanks in advance for help. Answer I have reviewed your circuit and the data you sent me. To start with, it is useful to look at the data you sent in terms of log conformance; i.e. the deviation from the expected logarithmic transfer function. If you look at the analysis tab on the enclosed spreadsheet, you'll see that the maximum deviation from the expected log characteristic is within +/-0.5dB for all cases, which is the log conformance spec for the AD8307. So the design to start with is not that bad. There are, however, a couple of issues when working with DC signals which you should be aware of. The inputs to the AD8307 are DC coupled so in theory, there is nothing to prevent you using the AD8307 to measure DC signals. The inputs to the AD8307 are fully differential and have a nominal common mode voltage of Vdd/2 (2.5V for a 5V supply). If you DC couple the inputs you need to take this biasing arrangement into account. Using the high side current sense circuit in your schematic, you essentially have a single ended source referenced to the 3V supply, the common mode voltage is also 3V. Any time you change the nominally 3V supply voltage you also change the bias conditions for the input stage of the AD8307 so you will see a change in intercept, slope or both. It will be important to keep the 3V line absolutely solid, we do not spec any common mode rejection for the AD8307 as it is designed as a IF power detector. The second thing to bear in mind is that the automatic offset null loop does not operate when the inputs are DC coupled. The offset null loop tries to remove any DC offset at the inputs and the circuit cannot distinguish between the DC input signal and the DC offset. It is therefore necessary to disable the offset null circuit by overriding OFS with a DC voltage greater than ?400uV. Offset adjustment is then performed manually either by applying a signal to OFS as described in p18 of the datasheet or you can use the AD8318 to level shift the signal, provide single ended to differential conversion and inject an offset compensation voltage as shown in figure 36 of the AD8130 datasheet. I've answered your specific questions below, but I suspect many of them are already answered from the discussion above. - Would you prefer the AD8310AR (has an additional output buffer and better datas than AD8307AR) or another device instead of the AD8307AR? The AD8310 has a voltage output and is faster to respond than the AD8307. But operating with DC signals you'll still need to take the same precautions - Do you see any possibility to reduce the big influence of the supply voltage (+3.3V)? I will try ones more to feed the circuitry separately. If you want better common mode rejection, use the AD8138 to level shift the signal and convert to differential form. - Is it necessary to shield the whole circuitry? If yes, this would raise the effort quite much. There is no inherent need to shield the AD8307 or AD8310, good layout and grounding should be enough. But if the part is expected to operate in a very noisy environment or the signal is likely to be contaminated by many ESD events, you might need to consider sheilding - The DC-voltages at the differential inputs is about +3V whereby the supply voltage for the AD8307AR is +5V. Does this work properly? In the datasheet is only mentioned that the DC level has to be min. 2V over GROUND. This works fine, but do not expect the AD8307 to insensitive to changes in the common mode voltage - Shall I provide a DC voltage at the OFS input or is it ok. Like I have done it? You need to disable the internal auto null circuits and manually compensate for offset using one of the methods described above.