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Hello people. I am an electronic student and i have been developing a test board for the AD8336 to control the gain of a signal. I am using a pre-amplifier set to 12 dB (4x) and a +-12 V supply. To control the gain it is supposed to use the GPOS and GNEG pins (VGAIN=VGPOS-VGNEG). One of the problems is that, even though i have for example 163 mV of VGAIN the amplifier does not amplify like Figure 78 of the datasheet says (in fact it doesn't amplify at all :S). You can look at the two images: one before powering the amplifier where the input of the amplifier is a perfect sinewave on yellow and the output on blue (not yet amplified); and the other image with the VGA powered where the input is distorted and with a DC component (not a sinewave) and the output has some small waves but no amplification is noticed.

The other problem is that i am using the configuration present on Figure 3 of the attachments (using a DAC to control the voltage of GPOS and with a fixed VGNEG, both of them preceded by voltage divider with 1% precision MOhms resistors) -> it's clear if you look at the image. The voltages on the control pins are not how i expected. Do this 2 pins (GPOS and GNEG) pull any current? Cause that is bad for the voltage divider topology i chose.

I have also checked if the VGA is properly powered with +-12V and GND.

Could you look at the my board schematics to see if you find any possible mistake?

André

attachments.zip
• Oh, i am going through the datasheet again and i noticed that there is a 453 Ohms resistor connected between the output pin and the oscilloscope. Is that needed??

André

• Hello James, thanks for the quick reply =).

I have some doubts about what you wrote.

1. Is my 'common-mode voltage' the average between the range of voltages i use? In my case i need gains between -14dB and 30.45dB. This means that the gain control voltage (V) are between -0.6 V and 0.28 V. The average of this range is -0.16 V. So, do i need a voltage reference of -0.16 V on the VCOM pin?

2. "Then adjust your divider total impedance around this  number and test a few to see if its suitable." Which number are you talking about? Do i need to consider the 1uA current and calculate maximum total impedance to ensure the 1uA for the voltage divider? In my case V=3.3 V, I=1uA so the sum of the resistors should be no more than 3.3 MOhms?

André

• Hi again James.

I understood almost everything now, apart from the 1st paragraph. My DAC topology can call for negative inputs. This is in fact true because the DAC output after the voltage divider (GPOS) minus the voltage in GNEG (VGAIN=VGPOS-VGNEG) can be positive and negative, since the fixed VGNEG can be higher or lower than VGPOS. So in terms of VCOM voltage - what is its value?

André

• Hello Andre,

Thanks for contacting us, I'm the applications engineer for the AD8336.

There are 2 problems with your circuit:

1. Although you provided an elevated reference for the GNEG input, you also need to provide a VCOM equal to the 'common-mode voltage' of the gain-control range of your drivr circuit. For example, if you have need the full gain range of 60dB cotrollable by only positive voltages, you'll require a 0C to 1.4V, and a common-mode voltage of 0.7V at the VCOM pin. Since you're using a DAC, you'll want to account for some minimum DAC range, e.g., 0.1 V. This makes your control range 0.1 to 1.5V, and the new VCOM will be 0.8V.

2. The second problem is the divider impedance. Note that the input current of the gain control interface is 1uA, typical. A typical specification means it isn't tested or controlled so you should anticipate some variation here. Our testers will no-doubt exercise this pin in production with buffers or op-amp circuits in which case the drive impedance is 0 ohms. I cannot predict what the variance in input current will be, but as a designer I might assume a large value just to be safe. Then adjust your divider total impedance around this number and test a few to see if its suitable.

You also asked about the resistor in series with the output. I usually provide resistors in the output of eval boards as a brute-force protection measure in case someone accidentally shorts the output or, like myself does something stupid once in awhile . Feel free to ignore it in the final design. Oh by the way it's a way to test the part using the 50 ohm load in a scope or piece of test equipment without unduly loading the output.

Good luck in your studies and best regards.

James

• Hi Andre,

Sorry no, I guess I wasn’t clear.

I take it from your schematic that you want to use the unipolar output from your DAC, so you cannot use a design calling for negative inputs. Also, consider the GPOS and GNEG inputs are differential, but relative to a ‘floating’ reference point. The reference must be considered if the gain control voltage function normally. You calculated a control voltage range of -0.6V to 0.28, or a range of 0.88V but since you need less than the maximum gain, you’ll need to provide a control voltage corresponding to left half of the plot in figure 3. Because there is more error on the gain limits, your common-mode supply ought to be 0.7V (for the AD8336) + the minimum DAC voltage. Assuming a 0.1v for the DAC, your VCM should be 0.8V

When using a Vcm circuit like this you can check out figure 82 in the AD8337 data sheet. The Vcm must be capable of sourcing and sinking current, thus must be a controlled voltage source such as an op-amp. To ensure low high frequency impedance, the pin will need robust by-passing. If you want to use fig. 82, note that the op-amp U3 can have almost any kind of reference, I used a regulator just because it was convenient 2.5V part that might be useful for an example using a single 5V supply.

As for the divider impedance:

Assume a maximum current of 10uA, then assume you want a maximum regulation of 1%, and a maximum load voltage of 1V. Superposition applies, so you can think about a simplified concept of a resistor, voltage-drop and current. One of the resistor is ‘grounded’, across the resistor is our 1% allowed regulation drop (or .01V) and the maximum current in/out of the Vgain is 10uA. Then the equivalent resistor value is 10k. If you had a divider of equal resistor value, each side you be 20k.

I hope this helps,

Let me know if you have more questions,

James

From: andrefleitao analog@sgaur.hosted.jivesoftware.com

Sent: Monday, May 09, 2011 10:37 AM

To: Staley, James

Subject: New message: "AD8336 VGA problems"

Analog Devices EngineerZone<http://ez.analog.com/index.jspa>

reply from andrefleitao<http://ez.analog.com/people/andrefleitao> in Amplifiers - View the full discussion<http://ez.analog.com/message/24722#24722

• Hi James. I am studying in Portugal in Instituto Superior Técnico.

Sorry for my doubts :S.

Yes the voltage in GPOS will always be positive indeed. I know these pins are not voltage sources. I am just asking about this sentence you wrote :

"You calculated a control voltage range of -0.6V to 0.28, or a range of  0.88V but since you need less than the maximum gain, you’ll need to  provide a control voltage corresponding to left half of the plot in  figure 3. Because there is more error on the gain limits, your  common-mode supply ought to be 0.7V (for the AD8336) + the minimum DAC  voltage. Assuming a 0.1v for the DAC, your VCM should be 0.8V"

I have an 8 bit DAC and 3.3 V supplying it so the minumum DAC voltage is 0.013 V - so the potential i have to connect VCOM is 0.713 V ?

André

• Hi Andre,

I was referring to the voltage developed at the DAC output, which will always be positive, according to your schematic. The GPOS and GNEG inputs are not voltage sources, rather they are inputs and won’t normally exhibit drive characteristics of either polarity.

BTW, where are you studying?

Regards,

James

From: andrefleitao analog@sgaur.hosted.jivesoftware.com

Sent: Monday, May 09, 2011 12:18 PM

To: Staley, James

Subject: New message: "AD8336 VGA problems"

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reply from andrefleitao<http://ez.analog.com/people/andrefleitao> in Amplifiers - View the full discussion<http://ez.analog.com/message/24728#24728

• Hi Andre,

You are correct! If you had identified the DAC I missed it or I would have looked up the part number. In lieu of the actual part number, I was presenting the idea as an example. Hence the word ‘assuming’. You are correct in using the actual data for the part you’d like to use.

Best regards,

James

From: andrefleitao analog@sgaur.hosted.jivesoftware.com

Sent: Tuesday, May 10, 2011 6:40 AM

To: Staley, James

Subject: New message: "AD8336 VGA problems"

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