DDS 50 ohms buffer ?

[Gillis]

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

 

[John Larkin]

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

Any ole fast opamp can do this. AD8014 or something.

John

 

[Fred Bloggs]

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

The AD9835 analog output is a *current* output DAC and that is why the
pin is labeled IOUT amazingly enough. The conversion is between a
digital evaluation of COS(phase) and the corresponding analog current.
The 300 ohm between IOUT and GND is used to perform a current to voltage
conversion. For best linearity Analog Devices recommends the full-scale
output current be 4mA and the maximum output voltage on the IOUT pin of
~1.2V. This boils down to driving REFIN with REFOUT, using RSET=3.9K
ohms, and terminating IOUT with 300 ohms DC. This will produce ~1.2Vpp
output centered on 0.6VDC. Assuming you want a pure ac signal on the 50
ohms lines with a gain of 8dB=~2.5 requires an amplifier capability of
6Vpp using the standard series terminated line driver. You can't do that
with a unipolar 5V supply.

 

[Mac]

Any ole fast opamp can do this. AD8014 or something.

John

Not all op-amps can drive a 50 Ohm load, though. Op-amps designed for
video can usually drive low-impedance loads, and might be a good choice
for this application.

Mac