Coupling and noise questions about THS7530

Hello Bo,

1: Figure 21 is labeled wrong, it is obviously not DC coupled, good catch.

2: Noise figure is not directly related to total SNR of the amplified signal. The noise figure compares the noise at the amplifier output with the noise at the amplifier input due to the source. The noise figure of 30 means that the output noise of the amp will be 30dB greater than it was at the input. This noise will add in a root sum square (RSS) manner with your signal noise. To know the SNR of the output signal you need to RSS the amplifier output noise with your signal noise. (uncorrelated noise sources do not add in a linear fashion)

3: Note that in the datasheet the input referred noise voltage is only specified at maximum gain. You probably cannot use this number at other gain settings.

4. This would suggest that this amplifier has constant noise voltage at the output over the gain settings. This would correlate with the noise figure getting 10dB worse for every 10 dB decrease in gain. Between Figure 3 and the TINA model I think that the amplifier is one with constant output voltage noise (approximately). This means that the output noise voltage is equal to 1.1nV/rt Hz amplified by 40dB which would be around 2mV in 300 MHz. So your SNR for your example would be 6mV / 2mV or 9.5dB. I cannot comment on the TINA simulations without seeing your schematic. Did you use a filter to make the 300MHz bandwidth?

Regards,
Loren

Hi Loren,

Thank you very much for your reply. I still have some questions:

1. If I want to use the THS7530 in "DC-Coupled Single-Ended Input With DC-Coupled Differential Output" configuration, do I need just remove the 0.1uF capacitor at the input port as shown in Figure 21?

2. You are absolutely correct about the RSS, but I still have difficulty to use the input noise, 1.1nV/rtHz. Could you show me an example in which the two methods (noise figure and input noise) agree with each other?

3. For the simulation, I did not use any filter and just set the frequency range to 1Hz to 300MHz.

4. Because the 5mV noise is so large, could you suggest some other circuits to achieve the gain adjustment and I want to use it to build a AGC?

Thanks.

Bo

Hello Bo,

Yes, for DC coupling remove the capacitor on the signal source.  Figure 17 shows a DC coupled circuit. 

What maximum and minimum gains do you require?  Do you want a continuous adjustable amplifier or a digital controlled amplifier?

 

Our VCA series may be more suitable for your application.  The VCA821 has user selectable maximum gain, so you can scale the max gain for your application.  It does not have the output voltage clamps,though. 

 

Regards,

Loren

 

Hi Loren,

Thank you for the suggestion, but I only have +5V supply on my board, so I used the THS7530, but encountered a problem. I set the VOCM=2.5V, but only get 2V to 3V swing range from the single output. But in the datasheet on Page 5, the Maximum output voltage high is 3.5V and the Minimum output voltage low is 1.5V, so it should give 2V swing range on each output pin and the differential should be 4V. So how can I correct it? I used the DC-Coupled Signle-Ended Input Width DC-Coupled Differential Output design on Page 14. But I did not use the two 1kohm resistors to get the 2.5V bias for the inputs, instead, I connected the two 49.9ohm resistors to a 2.5V voltage reference.

Thanks.

Bo 

Hello Bo,

What gain setting are you using on the Vg pin?  It may be that the amplifier gain is too low to get a full output swing. 

Regards,

Loren

Hi Loren,

The Vg+ =0, Vg-=0. But I can see the flat top of the amplified signal. Vcm=2.5V and Vout+ cutoffs at 3.0V and Vout- cutoffs at 2.0V.

Bo

Hello Bo,

What is the load resistance for your circuit?
Are you measureing the output swing at the amplifier output pins or after a matching resistor?

For example, in Figure 21, the 24.9 output resistors will cause some loss in voltage swing. The exact amount is dependent on the load resistance.

Regards,
Loren

Hi Loren,

The load is the 24.9 ohm in series with a 150ohm to the input of a differential amplifier, with 1kohm feedback.  I measured the output at the amplifier output pins.

I just found that when I add 150mV to the Vg+  to Vg-, the output can reach +/- 1Vpp, but it does not improve the already saturated output when the Vg+ to Vg- is zero. So the saturated output was +/-500mVpp and now it becomes +/- 1Vpp but it is still saturated.

Thanks.

Bo

Hello Bo,

How are you measuring the amplifier output? Are you using a FET probe? If you are using an oscilloscope, what is the input impedance?

There are two causes of saturation in an amplifier output, one is too much voltage, the other is too much current. It would be helpful if I had a schematic of the full test setup.

Regards,
Loren