Unless you are using something like a current-feedback op-amp, every op-amp will also present that BW vs gain compromise, that is not unique to the AD8129. Also, a CMRR of 40dB at 100MHz is actually really good, parasitics really make it hard to maintain equal common-mode impedances at such frequencies, and, at those frequencies, the PCB and design also has a huge impact. Also, why do you care about 1/f if you are in the RF range? You might want to consider a hybrid discrete + opamp solution, that way you can lower the gain of the AD8129 and share it with a low-noise input transistor.
To be honest your specs don't make much sense to me, you mentioned previously you wanted audio range, and RF starting at 1 kHz? I don't understand, that is not even close to RF, and now you mention 100 MHz. I think there is some miscommunication going on.
To be honest your specs don't make much sense to me, you mentioned previously you wanted audio range, and RF starting at 1 kHz? I don't understand, that is not even close to RF, and now you mention 100 MHz. I think there is some miscommunication going on.
Look, I mentioned 3 different LNA's to build, as each with specific performances.
Now the current task is to build a LNA RF amplifier up to some xxx MHz or below 1 GHz.
The task is to measure the RF ripple on digital or analog LDO's.
Required is a differential AC input, (+) to go to Vpp and (-) to go to GND close as to (+) connections as with a 2.54 pin header.
Additional grounding shielding as freely connected as a differential probe from LeCroy AP034 but powered with gain (may 20..30dB).
Now the current task is to build a LNA RF amplifier up to some xxx MHz or below 1 GHz.
The task is to measure the RF ripple on digital or analog LDO's.
Required is a differential AC input, (+) to go to Vpp and (-) to go to GND close as to (+) connections as with a 2.54 pin header.
Additional grounding shielding as freely connected as a differential probe from LeCroy AP034 but powered with gain (may 20..30dB).
Then, an op-amp is not the best solution for what you are asking.
You don't mention specific performances, you give figures like xxx MHz or below 1 GHz, that entails everything from 100-999 Mhz. Also, I gave you a recommendation on an op-amp with low 1/f noise, which is the first LNA you mentioned, yet, you said you are focused only on the RF LNA. Im am not sure what you want, but I think we are not going to reach an understanding.
You don't mention specific performances, you give figures like xxx MHz or below 1 GHz, that entails everything from 100-999 Mhz. Also, I gave you a recommendation on an op-amp with low 1/f noise, which is the first LNA you mentioned, yet, you said you are focused only on the RF LNA. Im am not sure what you want, but I think we are not going to reach an understanding.
Last edited:
The input voltage will be AC only. So to measure any RF ripple on 12V OXCO or on any ADC / DAC power.
I expect a maximal of 20...50mVpp ripple on those LDO's power output. The frequency base frequency of the MCK could be up to 100MHz as
some ADC/DAC clock needs.
For those high clocks as 100MHz, any 10 harmonics to measure. That's why 1GHz BW would be a nice to have.
This looks as a new field, as AKM & ESS & others do not specify the maximal ripple for those ADC /DAC chips.
I expect a maximal of 20...50mVpp ripple on those LDO's power output. The frequency base frequency of the MCK could be up to 100MHz as
some ADC/DAC clock needs.
For those high clocks as 100MHz, any 10 harmonics to measure. That's why 1GHz BW would be a nice to have.
This looks as a new field, as AKM & ESS & others do not specify the maximal ripple for those ADC /DAC chips.
Ok, things are making more sense now. What you want is not easy to do for several reasons:
- You mentioned 40 dB of gain, which is equal to a voltage gain of 100, that means you would need an op-amp with a Gain-Bandwidth product (GBW) of at least 100 GHz, AFAIK there is no such thing, but if someone knows of an op-amp with a GBW of 100 GHz, I would like to know. Even if you use two stages and split the gain, each with 20 dB, you would still need op-amps with a GBW of more than 10 GHz, since, in this case, the total attenuation at 1 GHz will be 6 dB not 3 dB. You might use more op-amps to increase the combined BW, but you would be also increasing noise, since the total Noise Figure deteriorates as you add more stages and the stages get reduced in gain. Op-amps are not high-frequency devices, so you are most likely going to have to resort to discrete components.
- Another problem is that 1 GHz is, in general, as high as you would like to go in an FR4 substrate, since the losses in the dielectric start becoming more relevant. Parasitics are also a big issue as frequency grows larger, which will deteriorate the CMRR and response of your circuit.
- If you want a proper amplifier with a relatively flat gain up to 1 GHz, you want to start thinking of transmission lines rather than PCB traces, this means that you need to be able to have a well-defined characteristic impedance for each line, and the specs of FR4 are not usually well-defined, plus, the permittivity of the dielectric in FR4 is most likely not constant or well-controlled. Typically, the permittivity in FR is around 4.5, but that is a ballpark figure, it could be considerably lower than that. If you need a high-end substrate for high frequencies you would have to buy a substrate from companies like Rogers, and they are most likely not going to sell it to you without a very good reason, so FR4 it is.
- There is the issue of impedance matching. At this point, I think it is not an exaggeration to state that what you are looking for is a low-noise microwave amplifier or at least very high RF (most people consider 1 GHz to be the dividing line between RF and microwave frequencies), so you may have to use RF techniques to design your amp, this means stuff like S-Parameters, impedance matching, Smith Chart, etc...
- At high frequencies amplifiers with negative feedback are harder to design, since we can no longer assume that a change of voltage/current in one part of a circuit causes an instantaneous voltage/current change in another part of the circuit. On top of everything there is the issue of stability, which is always a headache for RF designers.
So, to sum up, from what you have stated you want a high-frequency, low-noise, high and flat gain, differential RF amplifier with high CMRR and a 1 GHz BW. That is not easy to accomplish, not saying that it can't be done, but you can just go to Mouser or Digikey and buy an off the shelf part and drop it in a PCB willy-nilly. There are several considerations you should take into account. You may want to start looking for a high-frequency amplifier chip rather than an op-amp. AD sells some, but you can also look for products from high-frequency manufacturers like Mini-Circuits, they might have something that fits your needs.
- You mentioned 40 dB of gain, which is equal to a voltage gain of 100, that means you would need an op-amp with a Gain-Bandwidth product (GBW) of at least 100 GHz, AFAIK there is no such thing, but if someone knows of an op-amp with a GBW of 100 GHz, I would like to know. Even if you use two stages and split the gain, each with 20 dB, you would still need op-amps with a GBW of more than 10 GHz, since, in this case, the total attenuation at 1 GHz will be 6 dB not 3 dB. You might use more op-amps to increase the combined BW, but you would be also increasing noise, since the total Noise Figure deteriorates as you add more stages and the stages get reduced in gain. Op-amps are not high-frequency devices, so you are most likely going to have to resort to discrete components.
- Another problem is that 1 GHz is, in general, as high as you would like to go in an FR4 substrate, since the losses in the dielectric start becoming more relevant. Parasitics are also a big issue as frequency grows larger, which will deteriorate the CMRR and response of your circuit.
- If you want a proper amplifier with a relatively flat gain up to 1 GHz, you want to start thinking of transmission lines rather than PCB traces, this means that you need to be able to have a well-defined characteristic impedance for each line, and the specs of FR4 are not usually well-defined, plus, the permittivity of the dielectric in FR4 is most likely not constant or well-controlled. Typically, the permittivity in FR is around 4.5, but that is a ballpark figure, it could be considerably lower than that. If you need a high-end substrate for high frequencies you would have to buy a substrate from companies like Rogers, and they are most likely not going to sell it to you without a very good reason, so FR4 it is.
- There is the issue of impedance matching. At this point, I think it is not an exaggeration to state that what you are looking for is a low-noise microwave amplifier or at least very high RF (most people consider 1 GHz to be the dividing line between RF and microwave frequencies), so you may have to use RF techniques to design your amp, this means stuff like S-Parameters, impedance matching, Smith Chart, etc...
- At high frequencies amplifiers with negative feedback are harder to design, since we can no longer assume that a change of voltage/current in one part of a circuit causes an instantaneous voltage/current change in another part of the circuit. On top of everything there is the issue of stability, which is always a headache for RF designers.
So, to sum up, from what you have stated you want a high-frequency, low-noise, high and flat gain, differential RF amplifier with high CMRR and a 1 GHz BW. That is not easy to accomplish, not saying that it can't be done, but you can just go to Mouser or Digikey and buy an off the shelf part and drop it in a PCB willy-nilly. There are several considerations you should take into account. You may want to start looking for a high-frequency amplifier chip rather than an op-amp. AD sells some, but you can also look for products from high-frequency manufacturers like Mini-Circuits, they might have something that fits your needs.
Last edited:
Thank you!
Here some measurements on my differential amp-probe (AD8129) and I need to consider your writings!
While I simple expected, that there are any new & better AD8129.
Also an issue IMHO, is the harmonic phase shift (s), as looking on the square signal as may distorted by the phase shifts f (freq).
Here some measurements on my differential amp-probe (AD8129) and I need to consider your writings!
While I simple expected, that there are any new & better AD8129.
Also an issue IMHO, is the harmonic phase shift (s), as looking on the square signal as may distorted by the phase shifts f (freq).
