need ultraclean 3.3v 100ma DC supply for RF ADC - Page 1
Forum section: electronics / projects

author: cncjerry
author_rank: Unknown
timestamp: November 29, 2020, 11:27:00 pm
content: In another thread I'm building a phase noise test apparatus that will produce measurements in the -180dBc or less range. The oscillator for the 14bit, 125Msps ADC is an SMA connectorized module that needs 3.3V at 45ma injected into the opposite SMA connector. The original designer used a capacitance multiplier circuit followed by an 3.3V LDO coming off of an already clean 5V supply from the FPGA board. I have an lt1086ct-3.3 and a couple other 3.3v LDOs, all overkill for 100ma and probably not the best choice for this anyway.I'm hoping someone can point me at a circuit to provide what the original author wanted of "Ultraclean power". He had used a piece of rg213 from the power supply to the oscillator but I want to mount this on a small board with an SMA connector to attach directly. The ADC is running at either 77Mhz or 125Mhz depending on the DUT at the time.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: November 29, 2020, 11:54:37 pm
content: I'd start withLTC AN159 – Measuring 2nV/√Hz Noise and 120dB Supply Rejection on Linear Regulators, to at least see what you're in for in terms of "ultra-clean". Beyond selecting an appropriate regulator, it's basically a question of putting the right capacitors in the right places, possibly using tricks like capacitance multipliers.

author: cncjerry
author_rank: Unknown
timestamp: November 30, 2020, 01:09:06 am
content: thanks, exmad.I built that circuit in ltspice using my LDO and a double capacitance multiplier and then compared that to the LT3042 in that paper you attached. For input, I put 10mv at 120hz riding on the 5V DC. You can see from the plot that the lt3042 did a much better job at eliminating that frequency. Before making a decision I'm going to measure the 5V DC feed to see if there is a prominent noise frequency. Overall interesting paper.in case it isn't clear, the LT3042 is the green trace and the lt1086 with dual capacitance multipliers is the red trace.

author: David Hess
author_rank: Super Contributor
timestamp: December 03, 2020, 05:47:54 pm
content: I recommend some variation of the circuit below which allows Kelvin sensing to remove ground loops, a dedicated low noise reference, higher open loop gain by removing thermal feedback between the output stage and error amplifier, which allows better precision.The pass element can be an integrated regulator with its built in overload and thermal protection. Since the integrated regulator is within the feedback loop, its noise contribution is suppressed within the bandwidth of the feedback loop and can be ignored.Note that the example below is from decades ago and a modern implementation would use different parts, but the idea is the same. I prefer to replace the JFET with a PNP bipolar transistor and perhaps operate it at a higher bias. If a faster lower noise operational amplifier is used like the OP27 or LT1007, then frequency compensation needs to be added; I always layout complete type 3 frequency compensation but usually not all of the components are required.Watch out for the output range of the operational amplifier if a low voltage regulator is used. The PNP helps here a little bit by adding its 0.7 volt Vbe. For instance if an LM317Z was used, then the voltage at the adjustment terminal is only 1.25 volts below the output and conventional operational amplifiers will have difficulty driving that while the added Vbe helps considerably. In a low voltage application, a rail-to-rail output operational amplifier may be required unless higher supply voltages are available. Last time I used this circuit with a 7805 regulator to make a 10 volt output so the adjustment pin voltage was a convenient 5 volts.