OP295 inverting/non inverting amplifier

Hi Temistocle, 

Are you referring to the OP295 or the OP285?

In general for choosing resistors for your gain configuration, you will have to think about power and noise. 

In terms of power, the amplifier will see the feedback network as a load, so if you want to minimize the current draw through these resistors, you'll want to keep the values of the resistors sufficiently high - resistors in the range of 1 to 10s of kohms are common depending on whether you are keeping it low power or not. 

In terms of noise, the resistors will add thermal noise to the system. As an example, a 1kohm resistor will have 4nV/rtHz of noise. A quick look at the datasheets show that the OP285 is a 7nV/rtHz part, while the OP295 is higher noise at 51nV/rtHz. If you were using the OP285, the noise of a 1kohm resistor would start to be comparable to the noise of the op amp, whereas for the OP295, it would be negligible. That is something you will want to give some thought to. 

An additional note: the bias current of the amplifier will flow through the input resistor and result in an offset at the output. This should be considered as well. 

All in all, it really depends on what parameters you are shooting for. 

Does this help?

Thanks, 

Kris

Hi Temistocle, 

Are you referring to the OP295 or the OP285?

In general for choosing resistors for your gain configuration, you will have to think about power and noise. 

In terms of power, the amplifier will see the feedback network as a load, so if you want to minimize the current draw through these resistors, you'll want to keep the values of the resistors sufficiently high - resistors in the range of 1 to 10s of kohms are common depending on whether you are keeping it low power or not. 

In terms of noise, the resistors will add thermal noise to the system. As an example, a 1kohm resistor will have 4nV/rtHz of noise. A quick look at the datasheets show that the OP285 is a 7nV/rtHz part, while the OP295 is higher noise at 51nV/rtHz. If you were using the OP285, the noise of a 1kohm resistor would start to be comparable to the noise of the op amp, whereas for the OP295, it would be negligible. That is something you will want to give some thought to. 

An additional note: the bias current of the amplifier will flow through the input resistor and result in an offset at the output. This should be considered as well. 

All in all, it really depends on what parameters you are shooting for. 

Does this help?

Thanks, 

Kris

Hi Krisf,

thank your for the quick reply and for your help.

I need to use the OP295; sometimes the op amp datasheets contain also the recommended resistors values for buffers/inverters with gain +-1 (or +2), as for AD8001 (Table I at page 14 of its datasheet).

Temistocle

Hi Temistocle, 

Yes, the AD8001 would have these recommendations as it is a current feedback amplifier, and with this type of amplifier, the bandwidth is dependent on the feedback resistor used. 

The OP295 would be a conventional voltage feedback amplifier, and so the bandwidth is affected by the gain. In these cases, you have more freedom to choose your resistors, again based on the performance you want. For your case, you should be fine with 10k feedback resistors - low enough that it is lower than the noise of the amplifier and should not draw too much power. 

Thanks, 

Kris