THS3091: Inverting THS3091 Feedback resistors

Hi;

Correction, I made mistake with gains, it should be  x1 and x0.5 (not x2).

Regards;

Karo

Hello Karo,

   Did you also mean page 27 of the datasheet? Since you would use the THS3091 in an inverting configuration, it would actually be its negative counterparts in gain. If you look in the next page (page 28) figure 61, there is an example of the THS3091 in the inverting gain configuration with a gain of -2 which the resistor values are taken from Table 4. 

Thank you,

Sima

Hello Sima;

Thank you very much for the quick response. Apparently I was using older revision of the data sheet and table was in page. 17.

Does "gain of -2" mean that input signal is amplified by 2 and inverted and if I want to go with -0.5, then Rf=1K and Rg=2K?

Also for the unity gain invertor, can I go with Rf=0 & RG=2K?

Best Regards;

Karo

Karo, this is an current feedback amplifier so the frequency response is determined by the feedback R, 0ohm there will oscillate, 

What input impedance are you looking for? Often that is 50ohm, but for you - what is it? 

Make sure you look at the upgrade to this, the THS3491

Hello Michael; thank you very much for the response and THS3491 suggestion, most likely we'll go with THS3491.

Page 27 of the data sheet for the THS3091 has a table with suggestions for the Rf / Rg resistors for different gains. Is there such a table for the THS3491?

Input signal to the circuit can be either +/-10V with 10K termination or +/-5V with 50 or 75 Ohm terminations (jumper selectable) and we need to route this to circuit with MAX +/-2.5V input requirement, instead of voltage dividers we are planning to use THS3091 (or THS3491) in inverting mode (two of them to reverse the inversion), first with -0.5 gain and the second amplifier with jumper selectable -1 or -0.5 gain. between two amplifiers we'are going with 100 OHM termination. 

Using THS3091; can we go with Rf=1K & Rg=2K for the -0.5 gain and Rf = 1k & Rg = 1K for the unity gain? How about THS3491, will the same resistor values work too?

I attached partial schematic based on THS3091. 

Best Regards;

KaroInput Term.pdf

Well that schematic is showing a max +/-5V out of the 1st stage, The +/-15V solutions are kind of excessive power, 

You could do this with a+/-7.5V on the THS3491 or, +/-6.2V supplies on the OPA2677. The latter has 1.2V headroom over 0 to 70C, so+/-5V is possible and Absmax of 12.5V total supplies, so the 12.4V would be ok if you can control it tight enough. 

Need to make sure about part and supplies before Rf and Rg solutions. 

1. Desired flat bandwidth?

2. Desired large signal bandwidth.?

Hi Michael;

Thank you again.

We have +/-12V, +/-5V & +/-2.5V supplies in our circuit. Can we go with these voltages or we need to add +/-7.5V?  Will this work:1st Cascade +/-12V and second cascade +/-5V?

1. Small signal 200 MHz

2. large signal (+/-10V) 40 MHz.

Best regards;

Karo

Hello Karo,

    That is correct it would mean the input signal is gained by 2 and inverted. Michael's responses are very important to consider since as he said, this is a current-feedback amplifier. For the THS3491, which as Michael said is a upgrade to the THS3091,has a similar table located on page 25 with accompanying figure 62. However, this table will not be very helpful to your gain configurations since you would need a gain of -1 and -0.5 which are not listed in the table. This leads to the reason Michael would need to know your desired bandwidth since the stability, bandwidth, and peaking is dependent on choosing an optimal feedback resistor (Rf).

    Even though the voltage feedback amplifiers and current feedback amplifies have similar basic op-amp concepts, the value of Rf in current-feedback amplifiers is important to consider to achieve optimal performance. Choosing Rg comes later to specify the gain of the amplifier. There is a very helpful TI Precision Labs series on current-feedback amplifiers that you might find useful which will be linked here. 

Thank you,

Sima

I see a +/-10V input to an attenuator, but only +/-5V max output? The inverting input swing does not swing the input nodes so that is not an issue. 

Well if you want to stick with the +/-12V supplies, that is fine, Lets's assume the +/-5V max output is correct, at 40Mhz (sine wave?) that is 1256V/usce, so yes CFA is required. You have two stages, would you like to use a dual or two singles? 

Hi Michael;

We can go with a dual OPAMP, overall it is a 1x6 distribution with gain adjustments and since we are going to use the THS3491 on our output circuits to make up for the attenuation and get overall unity gain (+/-10V MAX), I thought we can go with the same amplifier.

Will the Rf / Rg resistor values in the schematic work?

Hi Sima, thank you for the pointing to the table.

Best Regards;

Karo

So lets start with the output stage, 

I think you are saying you are driving doubly terminated 50ohm lines in parallel, so +/-10V at the output pin becomes +/-5V at the matched load. Well 10V into 100ohm is 100mA for each line, that is 600mA total for 6 fan out. The THS3491 can probably do this, but I think the min spec is 500mA. May need to parallel output stages to guarantee 600mA if i I am doing this right. 

So you will want the THS3491RGT for best BW. you have jumpers for gain of 1 or 1/2. There should have been two tables by package over gain. Nominal gain of 5 for the RGT is 576ohm - I would guess (and you can simulate it) at low gains use 750ohm then the Rgs will be 750 ohm and 1.5k. 

Hi Michael;

You are correct about output levels and load, but 1x6 distribution and gain adjustments for each channel will happen before output amplifiers (THS3491) and we are going to have dedicated THS3491 for each output channel.

Best regards;

Karo

So in the prior stage you are running inverting gain of 1/2 w

With THS3491RGT, use 750ohm feedback and 1.5kohm input Rg from the terminations. Also, if you don't have to, don't load that stage with 100ohm - maybe more like 249ohm would save power. 

Hi Michael;

Thank you very much.

Regards;

Karo

Hi Michael;

Thank you again for all your help. We are going with RGT.

I just need a confirmation, can we leave PD pin floating?

Regards;

Karo

The backside metal can be floated (giving much higher thermal impedance) or connected to any supply or ground plane. 

Hello Karo,

   To add on to Michael's response, section 8.3 in the THS3491 datasheet will give important information on which pins can be left floated and which can not. The GND pin for RGT package cannot be left floated and must be connected to GND. The PD pin can be left floating; however, the datasheet states that TI does not recommend this in case stray signals couple into the pin which could lead to unintended turn-on or turn-off of the device. If you do leave the PD pin floating, it will float 2V below the positive rail which leaves the device enabled. 

Thank you,

Sima

Hi Michael;

I meant pin 16, ~PD.

Best Regards;

Karo

Hi Sima;

Thank you for the quick response.

I can connect either to +5V or +15V.

Does it matter which one?

Best Regards;

Karo

Hi Sima;

Thank you very much.

Best Regards;

Karo

Hi;

What resistor value I should use for a voltage follower in non-inverting circuit based on: THS3491RGT?

Best Regards;

Karo

Hi Michael;

Thank you very much.

Best Regards;

Karo