Use OPA695 for differential -to-single-ended DAC output amplifier

Hello,

That is certainly possible as long as it meets your output swing and distortion specifications. You will also lose the ease of the integration capabilities of the THS3215/THS3217.

Do you have a particular reason for choosing the OPA695 over the THS3215/THS3217?

The application note below shows how to use the OPA695 in a DAC differential to single-ended application.

www.ti.com/.../sbaa135.pdf

Hi Samir,

Thank you very much for your reply.

I believe that OPA695 is faster, cheaper and smaller.

Kind regards,

Bailin

Bailin,
The THS3217 OPS stage large signal BW of 500MHz vs the 450MHz of the OPA695. If you are trying to drive a large signal then the Large signal bandwidth spec (related to slew rate) is an important spec to consider. There are many reasons to go with the THS3217 or THS3215 over an OPA695 design. The THS3217 is a custom part that was designed specifically for DAC driver applications:

1. Ease of filter design - Most people use a filter between the DAC and the differential to single ended (D2S) conversion stage in order to reject the out of band images from the DAC. The design of this filter can become complicated, because of matching impedances of the filter and the resistors that from the D2S stage. The 1st stage of the THS3217 is a high input impedance stage which buffers the output of the DAC from the D2S conversion making the design simple.

2. It is difficult to get all the gain one needs from a single D2S stage so most people follow this up with a driver stage. The THS3217 has this driver stage integrated into it using the 2nd OPS stage. Note that good SNR performance another filter stage is sometimes added between the 1st and 2nd stage.

3. If the output common mode needs to be level shifted the THS3215/17 has a mid-scale low output impedance buffer which can be used for this level shifting.

The THS3217 combines these 3 amplifiers functions into a single device. if you don't really need to drive a large output signal or level shift the signal, then the OPA695 is smaller, otherwise from a system perspective the THS321x is the way to go.

-Samir

Hi Samir,

I agree that the D2S of THS3215 or THS 3217 is quite good. However, the minimum OPS gain of THS3215 or THS 3217 is 1.5. The ideal OPS gain is 2.5 at which most of its electrical characteristics are given. Therefore, the ideal total gain of D2S and OPS is 5. This gain is too high for our applications while we normally need a total gain of 2 to drive a 50 ohm load. Using the D2S to drive a 50 ohm load is insufficient. By the way, using its OPS to drive a 50 ohm load needs careful calculations of RG and RF in the similar way as calculating the resistor values for OPA695 presented in SBAA135, "Wideband Complementary Current Output DAC to Single-Ended Interface: Improved Matching for the Gain and Compliance Voltage Swing" by M Steffes. Please tell me what you think and any other similar components for our purpose.

Many thanks and kind regards,

Bailin

Hello Bailin,

 I have a couple of suggestions/inputs:

1. Using the D2S to drive a 50 ohm load - You are actually driving a total load of 100 ohms with 6dB of attenuation (50 ohm match). The D2S can drive a 100 ohm load without significant degradation in bandwidth (see Figure 14 of datasheet). From my experience the distortion of the D2S degrades by only 2-3dB under 100 ohm vs 200 ohm load.

2. Using the full signal path, i.e OPS + D2S - I believe you can actually do this and get your required gain. I would recommend adding a filter stage between the D2S and OPS (as shown in Figure 95).The interstage filter in figure 95 has a low insertion loss, but you can add more insertion loss by changing the value of the 464 ohm and 90.0 ohm resistors. Are you using a DAC to drive the amplifier? In that case you can maintain a full signal path gain of 5V/V and just reduce the DACs dynamic range.

We have a system level design of a DAC + THS3217 to help designers simplify/accelerate their design process. In case you are interested you can find the information here.

www.ti.com/.../tiduc44.pdf

-Samir