EMRFD Message Archive 8304

Message	Date	From	Subject
8304	2013-02-28 02:37:50	Alberto I2PHD	Need advice for interfacing an ADC
I know that in this group do lurk many expert RF designer, and I am seeking advice for a simple (for them) problem . I bought an inexpensive STM32M4F-Discovery board, with an ARM Cortex M4 processor which sports three ADCs on chip, that can be programmed to work in interleaved mode, reaching an aggregate sampling rate of more than 8 MHz. From the chip specs it looks like the input impedance of the single-ended S&H of the converters is around 10k to 50k. I would like to interface it with a 50 ohm antenna... which could be the best way to do this ? A step-up ferrite transformer with abt 1:15 turn ratio ? An op-amp ? Which one ? Preferrably not smaller than an SOIC... Given the sampling rate, the max input frequency would be not greater than 4 MHz. Advice welcome... thanks -- /*73 Alberto I2PHD*/ [Non-text portions of this message have been removed]
8305	2013-02-28 03:27:09	victor	Re: Need advice for interfacing an ADC
Most ADC's today work with a switched sampling capacitor input, so its not really important what is its input impedance because such an ADC requires to be driven from a low impedance source. Just look at the STM32F405xx datasheet page 126 equati
8306	2013-02-28 03:27:51	Lasse Moell	Re: Need advice for interfacing an ADC
Not knowing what full scale input etc you do need, I would look as a starter terminate input with 50 ohm and use one of those interface op-amps that are developed to drive A/D-chips. If you need to reduce the strong signal handling of interface you could try a 1:4 transformer allowing for 200 ohm input at A/D Remember light years ago, we had a discussion with then the state of the art SDR designer "Steinbrecher" and they used 1:4 transformer 50 to 200 ohm, but put a lot of design in the transformator to avoid IMD. If you want to match the three inputs, you will likely have problems to match the input with high ratio transformer, and they are often very narrow band too. Doesn´t ST Microwaves offer any recommended input design? /Lasse SM5GLC On 28 feb 2013 11:37 "Alberto I2PHD" <i2phd@weaksignals.com> wrote: > > > I know that in this group do lurk many expert RF designer, and I am > seeking advice for a simple > (for them) problem . I bought an inexpensive STM32M4F-Discovery board, > with an ARM Cortex M4 > processor which sports three ADCs on chip, that can be programmed to > work in interleaved mode, > reaching an aggregate sampling rate of more than 8 MHz. > > From the chip specs it looks like the input impedance of the > single-ended S&H of the converters > is around 10k to 50k. I would like to interface it with a 50 ohm > antenna... which could be the > best way to do this ? A step-up ferrite transformer with abt 1:15 turn > ratio ? An op-amp ? > Which one ? Preferrably not smaller than an SOIC... > > Given the sampling rate, the max input frequency would be not greater > than 4 MHz. > > Advice welcome... thanks > > -- > /*73 Alberto I2PHD*/ > > [Non-text portions of this message have been removed] >