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Wideband Loop Transimpedance Op Amp Selection

wideband loop transimpedance op amp

You can select candidates for a wideband loop transimpedance op amp by reading data sheets and simulating performance in LTSpice. 

One of my design criteria was to use operational amplifiers. Another was parsimony. I wanted to build a system with minimum complexity and parts count. So, time to read some data sheets.

You will find many transimpedance op amps. However, most of them are just designed for fairly low frequency uses, particularly converting currents from photo diode sensors. Tons of those. For my system, though, I need an op amp with a gain bandwidth product of at least 500 MHz in order to produce some decent gain over the entire HF range. Other factors include high frequency stability and reasonably good noise performance. Finally, the implementation must provide me with a very low input impedance when the TIA is inserted into the loop antenna.

My research produced two candidates, the Analog Devices LTC6629 and Texas Instruments LMH6629. So I attached models for each to my loop model in LTSpice and simulated performance. As you can see above, both TIA provide low input impedance when inserted into my loop. Performance is comparable to the LZ1AQ amplifier circuit, slightly better at the lower frequencies. I calculated input impedance as the loop voltage (V) divided by current flowing through the TIA feedback resistor. A good process for evaluating TIA input impedance is described here.

As you can see in the inset graphic, both of these devices appear to provide flat current-to-voltage conversion up to 100 MHz. This is more than I will need, so I need to find a way to roll off the gain or insert a low pass filter to attenuate above my design range, and in particular get rid of large FM broadcast signals which might cause IMD.

Wideband Loop Transimpedance Op Amp – LMH6629 Selected

In the end, I selected the TI LMH6629. You can see in the data sheet it is low noise (8 dB NF), high gain and bandwidth, and appears to have good dynamic range and output intercept specs. Since it will be mounted outside, it’s -40C temperature range will be useful. Like most TIA, it has the required low input capacitance and high input resistance. These are important for stability and ensuring the antenna current flows properly through the feedback resistor.

One other thing I liked about the LMH6629 was that it appeared easy to do a single supply implementation. If I can run my system off a single 5 volt supply (rather than split ±5) life might be simpler.

If you want to do the simulation yourself, you will need to download the SPICE model for this device.

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