What you always wanted to know about TINA-TI but were afraid to ask! (Part 3)

Whether or not you were ever “afraid” to ask about any of the TINA-TI topics we discussed in Part 1 or in Part 2 of this Analog Wire series, I’m hoping that you find my next topic, noise analysis using TINA-TI, useful in your day to day work.

Below are some of the TINA-TI noise simulation features which make it a great analysis and optimization tool:

1. Output RMS noise plot over any noise bandwidth.

• Integration “lower” and “upper” frequencies are entered when noise analysis is initiated.
• Use cursor(s) to read RMS noise over any bandwidth.

2. Noise density plot- referred to either input or to the output.

• Input referred noise density, when compared with signal power, lets you observe SNR directly.
• Output referred density plot to uncover unexpected peaking which could impact noise.

3. Noise gain frequency dependence is automatically factored in.

• For example, a Transimpedance amplifier where the photodiode capacitance affects noise gain.

The requirements for running noise analysis in TINA-TI are listed below:

1. One (and only one) source designed as input. Can be either voltage or current source!  More than one source situation covered below.

2. At least one output node.

3. Active devices macromodels which include noise behavior.

• Automatically done in TINA-TI when a part is placed on the schematic.
• Making sure modelled noise matches datasheet is covered below.

4. Start and stop frequency to be specified

• These entries define the range of noise density x-axis plots but also determine the output RMS noise integration bandwidth.

Figure 1 below shows the noise analysis panel and where the start and stop integration frequencies are entered.

Figure 1: Noise analysis panel & required frequency entries

 Figure 2 shows the use of cursors to find noise over a narrower band. An example of where this may be useful is to see how much noise can be lowered by filtering unwanted bandwidth.

Figure 2: Total noise over any bandwidth using cursors

If your circuit requires the use of multiple sources, simply follow the instructions in Figure 3.

Figure 3: Multiple source strategy for noise simulation

If you doubt how accurate your device noise model is, use the simulation circuits in Figure 4 to generate the plots (Figure 5) which you can then compare with the datasheet:

Figure 4: Uncovering the modelled noise of a device

Figure 5 shows the plots generated by noise-simulating the Figure 4 circuits. Results confirm good agreement with LMH6629 datasheet plots!

Figure 5: LMH6629 TINA-TI noise model matches well with datasheet

In the course of noise analysis, when curious whether the thermal noise (Johnson noise) of any of your resistors is dominant, replace that resistor with Figure 6 circuit (or its Macro provided in link further below). If you find that, as a result, output noise is reduced, then your circuit noise can be improved by lowering that resistor value (and thereby its thermal noise)! This is viable in many amplifier circuits as circuit operation is not affected if resistor ratios are maintained while the resistor values are lowered for lower noise.

Figure 6: Create a noiseless resistor, that you can place on your schematic, to evaluate thermal noise impact

Click here to get the TINA-TI macro using the Figure 6 technique. You can copy and paste this macro in any of your TINA-TI circuits to test for thermal noise (Right click on Macro, select Enter Macro and change “HCCVS1” from 10k to any resistor value you need, as shown in Figure 7):

Figure 7: Noiseless Resistor Macro (how to edit resistor value to what you need)

I’ll wrap this post up by providing a list of suitable devices for your next low noise design. I have included a column in Table 1 called “Critical Resistance” as the value of source resistance beyond which input noise current dominates over input noise voltage. When your source resistance exceeds this critical resistance, it is likely that you can benefit from changing your device to one with a lower noise current. The good thing is that you have TINA-TI at your disposal, to quickly change your device to another type and to run new simulations to verify lower noise.