How to pair a variable gain amplifier to an ADC - Page 1
Forum section: electronics / beginners

author: whatsthegeek
author_rank: Contributor
timestamp: October 09, 2019, 11:41:45 am
content: Hi, I'm currently designing my own Software Defined Radio. I'm already done with most of the front end but now I have to design the ADC and VGA circuitI decided to use a LTC2205 ADC and a LT5554 VGA, the specs seem perfect for what I want to do, however, I don't understand a thing to how I'm supposed to pair them upFirst of all, I need to convert the single ended signal to a differential signal with a bias voltage for the VGA. I know this should be easy to do with a 1:1 transformer with a center tap on the secondaryBut here comes the part where I'm lost:The VGA apparently needs some sort of constant current at the output?The datasheet says:"A transformer with a center tap tied to VCC or a choke inductor is recommended to conduct the DC quiescent current"What does that mean ?Also, the output of the VGA and the input of the ADC don't have the same impedance, how can I match them correctly to get the best results possible?And if that wasn't enough, the ADC needs a bias voltage on the inputI's also very important that the matching circuit works from as close to DC as possible, to at least 30MHzI'm pretty much completely lost at this pointAnd if you couldn't tell already, I'm a complete n00b at ADCs and RF. It's my first project even with both of theseSo, could someone help me understand why the VGA needs a bias current and maybe suggest a circuit?Thanks in advance

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2019, 02:53:20 pm
content: I also have no experience with RF ADCs or VGA circuits, but have quickly read the datasheet for the LT5554.It's a transconductance amplifier, which means it takes a voltage and converts it into a current. To convert the current into a voltage, a resistor is normally added to the output, but in this case, it has an internal 400R resistor connected across the output.There's no need to match the impedances, as long as the traces on the PCB are kept short enough. If the the traces between the ADC and VGA are no more than a couple of cm, then it shouln't be a problem.

author: whatsthegeek
author_rank: Contributor
timestamp: October 09, 2019, 04:10:32 pm
content: Thanks for the reply!So, all I would need to do to connect the LT5554 to the LTC2205 ADC is use two coupling capacitors and a 1:1 transformer with a center tap to bias the input?

author: Bud
author_rank: Super Contributor
timestamp: October 09, 2019, 05:15:24 pm
content: Quote from: Zero999 on October 09, 2019, 02:53:20 pmThis is incorrect, the mismatch will not go anywhere. I would not bother matching 1:1.5 SWR but if it is like 50 Ohm one stage and 200 Ohm the other stage that have to be matched.

author: whatsthegeek
author_rank: Contributor
timestamp: October 09, 2019, 06:16:47 pm
content: Yikes this stuff is hardIf I understand the datasheet of the ADC (which I'm not 100% sure I am), the manufacturer recommends a 50Ohm impedance but they don't say what the impedance of the ADC is.and, in the datasheet of the VGA, they say that output impedance affects the gain and SNR, so I better get it rightI'm even more lost at this point

author: Zero999
author_rank: Super Contributor
timestamp: October 10, 2019, 08:17:31 pm
content: Quote from: whatsthegeek on October 09, 2019, 04:10:32 pmNo biasing is necessary. The LT5554's inputs are internally biased to the appropriate DC voltage.Quote from: Bud on October 09, 2019, 05:15:24 pmWhat do you mean?I thought that depended on the frequency? The original poster hasn't said, but the LTC2205 data sheet talks about 700MHz, which is a wavelength of just under 43cm, so I think 2cm is reasonable not to worry about matching.Quote from: whatsthegeek on October 09, 2019, 06:16:47 pmThe LT5554 has an output impedance of 400Ohms and the LTC2205 and input impedance of 6k, so yes it's unmatched but for such short traces it shouldn't be a problem.