AD734 multiplier/divider - operates best well below minimum supply voltage ??? - Page 1
Forum section: electronics / projects

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 15, 2018, 12:36:49 am
content: Hello,I have built a multiplier circuit using the AD734 multiplier/divider (spec sheet here). My goal is to demodulate a nominally 5 to 30kHz signal from an avalanche photodiode. I'm testing the circuit with the outputs from a 2-channel DDS. By default, the AD734 applies a 10V denominator to the signal but I want 1V. So, I have implemented the circuit on page 12 of the spec sheet with the resistor values specified for a 1V denominator .I was able to get an appropriately demodulated signal. I used a passive LP filter on the output (RC ~1ms). The amplitudes of the input signals were 1 to 2V so I would have expected an output signal at least of the order of a volt. However, I only got a few 10mV and I noticed that there was a very narrow bandwidth for the input signals. All of the graphs in the spec sheet indicate performance should pretty much constant below 1MHz but I saw significant roll-off below 5kHz and above 100kHz.After scratching my head, I discovered a most odd thing. The operating supply is specified to be +/-8V to +/-16.5V but I was able to get an output of the order of a volt using +/-3VHow can that be so?Increasing the supply above +/-3V causes a rapid decrease in the output. I use a bench-top supply.I don't think I'm operating the AD734 at extreme conditions. Note, I have two identical circuits set up (on a solderless breadboard) and they behave exactly the same.I will try the fixed denominator (10V) circuit (fig. 24 on page 12) but I wanted to ask if anyone has any experience with this chip and/or if my symptoms suggest something else commonly encountered in electronics design.Thank you,John.

author: duak
author_rank: Super Contributor
timestamp: April 15, 2018, 04:22:37 am
content: I've used the AD734 for a few circuits, one being a real time power meter where it worked far better than I expected it to. I haven't used it for a modulator though. One thing I remember was having to scale the inputs and the outputs carefully so it either wouldn't clip or one of the terms being too low and then introducing errors or noise.If my memory of how it works is correct, linear input voltages are converted to logarithmic currents which are added and subtracted and then converted back to a linear voltage ouput. Adding or subtracting log currents is equivalent to multiplying the corresponding voltages so you get a multiplier divider. I'd bet if the AD734 is operated a reduced voltage some parts of it stop working as designed and introduce a nonlinearity or imbalance that is passed on the output.I'm assuming you're using the AD734 as a synchronous demodulator? Are you using two to make a quadrature (I-Q) demodulator? Although I think I understand what you've got, would you be able to give us a schematic with the signals you are applying and observing?Cheers,

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2018, 09:22:33 am
content: Where did you buy the AD734 from? It's an expensive part, so unless it's from a reputable supplier, such as Digikey or RS Components, then it's extremely likely it's fake.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 16, 2018, 02:36:39 am
content: Hi - I bought two of them from Mouser.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 16, 2018, 02:53:42 am
content: Thank you for your reply. (I forgot to check the "notify me of replies" option.)I am indeed planning to use two AD734's for quadrature demodulation.I was going to create a schematic (which is the same as in the spec sheet) but first I decided to set up the circuit on a separate, dedicated breadboard using the internal denominator option. Following the sheet, it didn't work but by leaving pins 3, 4, and 5 unconnected instead of grounded it did. I'm not totally convinced that the behavior of the chip is how it is described. I was very pleased with the quality of the demodulated/filtered signal. I changed the layout to the external denominator and, again, I could get it to behave (sort of.) I added an identical circuit to the same breadboard and the behavior changed. The first chip exhibited the same supply voltage dependency as before but the second didn't. A short while later, I obviously fritzed the first chip based on a comparison of pin-to-pin resistances for each chip. Ouch. I'm not surprised since I'd been removing/adding components for half the day troubleshooting. I guess my ESD precautions need upgrading.So, for the moment, this particular part of my project is on hold...John.

author: duak
author_rank: Super Contributor
timestamp: April 16, 2018, 02:57:12 pm
content: I reviewed one of my designs using the AD734 and see that I tied DD to Vcc (+15 V). If I read the pin description correctly, if DD is tied high the internal denominator value is disabled and U0 - U2 are enabled. If DD is floating, as in fig 24, the denominator is set to 10 V. Is this what you observe?Cheers,

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: April 16, 2018, 05:40:17 pm
content: Hi - thanks for looking into this.I rebuilt the fig 24 circuit and, at first, still didn't seem to get the right behavior. After more fiddling, I had an a-ha moment. The amplitudes of my X and Y inputs were rather small. I'd been testing with 0.1V 10kHz sinusoidal signals. Well, 0.1V X 0.1V / 10V is just 1mV. Applying 5V on the other hand should give 2.5V. The output amplitudes I get suggest a denominator value of 8V (not accounting for any attenuation from my output passive LP filter.)My photodiode signal passes through a +40dB gain 8-pole band pass filter and I should get signals of the order of 100mV. Of course, now I realize I can use a large reference (say 10V) which, with the default 10V denominator, should give an effective gain for the demodulation of 0dB.John.