PGA/VGA with ac-coupled bipolar input swing

Look at the VC820 which has 4 members in that family, and then there a string of multichannel CMOS parts for ultrasound like the VCA2612

Hello,

  I agree LMH6401 would be overkill for your frequency range. Additional fully differential PGA/VGAs to look at for your gain range: 

1. VCA2617: −10dB to 38dB, or −16dB to 32dB; dual channel fully differential VGA, 50MHz, +5.25 (+/-2.5V) supply

2. LMH6514: -12dB to 30dB, fully differential DVGA, 600MHz, +5.25 (+/-2.5V) supply

3. VCA2615: varying gain ranges between -15dB to 60dB (see in figure below, dual channel fully differential VGA, 42MHz, +5.25 (+/-2.5V) supply

Thank you,

Sima

Thanks for the reply.

VCA820 and 4 similar parts are not fully differential as the output is single ended.

VCA2612 and similar parts work on a single positive supply and don't accept the negative input range required. 

Thanks for the reply.

I have checked the datasheets of the parts suggested, but there is no mention of +/-2.5V supply.

In case I missed it, can you please point out where it is mentioned.

Hello,

   You are correct, I mistakenly thought these devices allowed dual supplies. Unfortunately, VCA2617, VCA2615, and LMH6514 only allow 5-6V single supply. Since this is AC coupled input application, would you be able to use a balun, and then for differential output you would need two amplifiers for differential output, like this app note as an example. However, obviously this will increase cost, space, and distortion.

   There is also the PGA870. This device is fully differential VGA, will work for split supply, but is 650MHz and gain range between -11.5dB to 20dB gain. So, this device will not have figures for your frequency range. But there is a Tina-TI/Pspice model and EVM available for this device to simulate/test for your frequency range. 

Thank you,

Sima

Ok, your 100kHz is pretty low for this function, but here is a possible solution

The VCA810 has the range but needs a post FDA to go back to differential out with fixed gain of 20dB - I used the THS4561 a +/-5V capable very low power FDA. 

The VCA810 was a process migration of the earlier VCA610 - in that era, diff in to single out was the need. so, in your case, AC couple to an FDA to add 20dB gain shifting the range up to where you want it and get back to diff out. This is a very large adjust range, normally you 

1. run into to input noise floor issues at max gain

2. run into input overdrive issues at min gain, but 

here you go, 

And a V9 version of this file, 

VCA810 with THS4561.TSC

And, now that I have an FDA, added a 150kHz 2nd order Butterworth. Not hitting that exactly, but this will bandlimit the noise, 

VCA810 with THS4561 with 150kHz Butterworth MFB.TSC

Thanks for the Reply.

PGA870 is a good suggestion, completely missed the part about dual supply in the datasheet.

Will check out the spice model. Only issue maybe to achieve the required gain, i may have to go with 3 stage of pga.

Thanks for the reply.

This is one of the solutions I was also looking into. I had not implemented the 2nd order butterworth, it is a nice addition for noise, will compare the noise with a fully differential VGA and decide.

Actually I had decided to go with the digipot initially, but there were phase issues with them. So decided to change as the application requires minimum phase distortion. I have checked the simulation file you provided without the filter, there was some minimal phase distortion which was observed. Do you have any suggestion to correct the same?

Hello,

  Using a fully differential amplifier, minimizing output voltage swing, increasing output loads, and careful PCB layout will improve phase distortion. I would set the second stage FDA (non gain control) to unity gain since you are looking to use a PGA that allows gain control lower than 0dB. 

Thank you,

Sima