Hi.

I try to figure out what can be the amplifier that can match the AD9835 300

ohms output impedance to 50 ohms with a gain of about 8 db at 13.5 Mhz.

Is this can be done with a simple transistor or a monolithic amplifier ?

I have take look on the net and they always use it unbuffered !

Thanks in advance for any information.


Gillis

Gillis,
Most of the Analog Devices DDS chips have a current-source output. This
means the DDS source impedance is set by an external load resistor (Rout),
making it somewhat arbitrary. I'm using 50 ohms in my unit. The peak RF
output voltage will be Idac x Rout, where Idac is the DDS DAC current.

As far as an amplifier is concerned, the choice is somewhat dependent on the
application. If the load circuit is fairly 'stiff' and is not real sensitive
to the source impedance, you can use a MAR monolithic amplifier
(Mini-Circuits or Avantek). However, these can become unstable when driving
reactive loads, and they also have poor reverse isolation, which means that
load changes could affect the frequency response of the DDS lowpass filter.
Also, you want good linearity, otherwise the waveform will become distorted
and the harmonic content will increase.

You might want to look into some of the ultra-wideband opamps that are
available from people like National Semiconductor.
Joe
W3JDR


"Gillis" wrote in message
om...
Hi.

I try to figure out what can be the amplifier that can match the AD9835
300

ohms output impedance to 50 ohms with a gain of about 8 db at 13.5 Mhz.

Is this can be done with a simple transistor or a monolithic amplifier ?

I have take look on the net and they always use it unbuffered !

Thanks in advance for any information.


Gillis

Gillis,
Most of the Analog Devices DDS chips have a current-source output. This
means the DDS source impedance is set by an external load resistor (Rout),
making it somewhat arbitrary. I'm using 50 ohms in my unit. The peak RF
output voltage will be Idac x Rout, where Idac is the DDS DAC current.

As far as an amplifier is concerned, the choice is somewhat dependent on the
application. If the load circuit is fairly 'stiff' and is not real sensitive
to the source impedance, you can use a MAR monolithic amplifier
(Mini-Circuits or Avantek). However, these can become unstable when driving
reactive loads, and they also have poor reverse isolation, which means that
load changes could affect the frequency response of the DDS lowpass filter.
Also, you want good linearity, otherwise the waveform will become distorted
and the harmonic content will increase.

You might want to look into some of the ultra-wideband opamps that are
available from people like National Semiconductor.
Joe
W3JDR


"Gillis" wrote in message
om...
Hi.

I try to figure out what can be the amplifier that can match the AD9835
300

ohms output impedance to 50 ohms with a gain of about 8 db at 13.5 Mhz.

Is this can be done with a simple transistor or a monolithic amplifier ?

I have take look on the net and they always use it unbuffered !

Thanks in advance for any information.


Gillis