Programmable gain amplifiers

[charliecoutas]

Has anyone on the forum ever used a MAX9939 programmable gain amplifier? I am including a few in a design and would appreciate any feedback. I am a little nervous about a total output swing of only 5 volts; I am happier with a 10 or 15 volt swing but I can't find any PGA's with a wider swing. It will of course be controlled by Positron software!

Best to all
Charlie

[rick.curl]

Hi Charlie-
How much swing do you need?  THS7001 will go up to +_ 13.6 volts.  Maybe a better question is what are your supply voltages?

-Rick

[charliecoutas]

That looks good Rick but I need gains of 1,10,20 (the THS7001 has non-integer gains).

I have +/-15v rails. My worry is that the MAX9939 biases the signal to be signal plus rail/2 internally. This limits my voltages in and out to 2 volts and I am worried that noise will be troublesome. I also like the idea of an SPI interface to set the gain. The MAX9939 also has a nice feature where you can adjust the offset of the amplifier using the software interface.

Thanks Rick
Charlie

[GDeSantis]

Two programmable gain instrumentation amplifiers from Analog Devices that may work in your application are noted below:

   1.  AD8250 with gain settings of 1, 2, 5, 10
   2.  AD8253 with gain settings of 1, 10, 100, 1000

[charliecoutas]

I can probably use one of each, excellent, many thanks.

Charlie

[top204]

I had a similar requirement about 12 years ago, and I could not find a programmable gain amplifier with enough current or voltage, and enough resolution so I could change the gain in small steps, so I used a standard high current/higher voltage Op-Amp, and a 256 step digital resistor for its gain. Together, they cost a lot less than some PGA devices at the time, and they were simplicity itself to design for.

Depending on the circuit's requirement, the digital resistor can be wired as a standard volume control or used for the gain resistor in an Op-Amp inverted or non-inverted amplifier.

[charliecoutas]

Thanks Les. I did think about a resistor-tree arrangement but space is a bit tight. To have a PGA with gains of 1,2,5 and 10 is perfect and the AD8250 also has differential inputs which solves another problem. Thanks GDeSantis.

But I will still use a PIC to control the gain settings!

Charlie