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Wilson,

60mV is most likely fine, but it depends on what your supply voltages are.

According to page 3 of the VCA824 datasheet, given +/-5V supplies, the most negative input voltage is -2.1V and the most positive is 1.6V. That is the most negative voltage should be 2.9V above the negative rail and the most positive voltage should be 3.4V below the positive rail.

Regards,
Luke Lapointe
High Speed Amplifiers

Hi,

I have my VCA824 on +/-5V supply voltage. But the waveform is clipped when the input source is:

Waveform type: Sine-wave,10KHz

Vpp : 60mVpp

I think I will do a check on my physical hardware again. Thanks.

Wilson,

I think you are on the right track to check the hardware first. I would also use a multimeter and check the DC voltages as well as the impedance around the board to make sure everything is connected how you expect. Sometimes there is a loose wire or connection that can make the circuit act unexpectedly.

If everything looks correct and you still see the issue, then you can upload your schematic and we can take a better look.

Regards,
Luke Lapointe
High Speed Amplifiers

Hi,

We had got it figured out after some testing.

By setting my desired A_v = 40V/V, R_G=18R and R_F=402R. My Vin(Vpp) is limited to around 100mVpp. This is due to the characteristic of the VCA which followed the following equation: Vin(Vpp)=2*R_G*I_RGMAX

Right now, I'm preparing to change my R_F and R_G to get a higher input dynamic range. 

Can I know if there is any relationship between input frequency with the R_F and R_G?

Thanks.

Wilson,

Your motivation for changing Rf and Rg to "get a higher input dynamic range" is unclear.  With Rg set to 18ohms, as you describe, you have Vin(Vpp) = 2*R_G*I_RGMAX = 93.6mV.  In a system configured for A_vmax = 40, this corresponds to a maximum output voltage of 3.74V, very close to the maximum capability of the device.  If you were to increase the input voltage range, without lowering A_vmax, you will begin to clip on the output, similar to Figure 72 of the datasheet.  So any attempt to increase the input dynamic range should be coupled with a decrease in A_v, in order to remain within the output voltage limitations of the device.  These and other design considerations are summarized on page 25 of the VCA824 datasheet.

Regarding Rf and Rg versus frequency, since the output stage of the device is a current feedback amplifier, higher values of Rf result in continually lower bandwidth.  To achieve frequency performance similar to that in the datasheet, it is recommended that Rf not exceed 500ohms.

Hi,

Thanks for your reply. Actually, I do require a large input dynamic range of 0 to 2Vpp.

The VGA design should ideally be able to amplify the signal when it is small and attenuate the signal when it is large. As the output of the VGA is 'piped' into the ADC, I will need to constantly maintained my output of VGA to be around 1Vpp to make use of the full ADC range.

However, I realised that the performance of VGA degraded when the input dynamic range is set to 2Vpp.

Can I know TI recommendation for Rf and Rg value to achieve a 2Vpp? with Av from 0V/V to 20V/V (min)? 

Or is there any other suitable chipset which you will recommend?

Thanks.

P/S: My signal frequency is approx100MHz

Hi,

I've jumped into this rather late. But, here is my 2 cents anyway!

I'd pick the value of RG to make sure I won't be limited by I_RG with your 2Vpp input. That means RG= 1Vp / 2.6mApp = 385ohm.

Set RF=500ohm to get the best speed / frequency response. Add another amplification stage to increase the gain to the 20 V/V that you are looking for.

Hope this helps.

Thanks,

Hooman

Hi,

Thanks for your reply. 

Can I know TI recommendation on RG and RF (based on VCA824) for the following:

(1) 0V/V to 1V/V

(2) 0V/V to 20V/V

If it is possible, can TI  provide the input dynamic voltage range and expected bandwidth for setting (1) and (2)? Also, will like to know if the linearity for setting (1) will still hold?

Next, do TI or NS has any VGA product which can meeting the following requirement:

Input Voltage Range : 5mV (small signal) to 2V (large signal)

Output Voltage Range : 900mV (small signal to amplify as much as possible, large signal to attenuate)

Dynamic Gain Range : 0V/V to 40V/V (Linearity is the main priority, follow by the maximum gain)

Signal Frequency : approx. 100MHz

Control via : Analog voltage (Control algorithm will come from my uController and DAC)