# DC PD for OMC

## OMC DC PD amplifier analysis

The full package for the analysis of OMC DCPD amplifier (LIGO-D060572) with various transimpedance (Made by Koji Arai)

### Method

The circuit model in LISO was made. The transimpedance resistor (rT) was varied from 10 Ohm to 3000 Ohm with a half-decade log-spacing (i.e., 10, 30, 100, ...). The transfer function (total transimpedance gain) in Ohm (=V/A), the input-referred current noise in A/rtHz, and the max input current range to have no saturation, in A, were calculated in each case.

### How to interpret the plots

• transimpedance.pdf: This is a basic check to see how the input current is converted to the output. At DC, the transimpedance gain is rT*2 because of the differential output. Between 100-10K, the gain is boosted to rT*(11^2).
• input_current_noise.pdf: The input noise is 20pA/rtHz (= the shot noise of iDC~1mA) when rT is 100Ohm. When rT is smaller than r8 (100Ohm), r8 cause the dominant thermal noise. When rT is larger, it dominates the noise. However, above rT=1kOhm, the input current noise of the 1st opamp dominates the noise. That's why the improvement of the input referred noise for rT=1k, 3k is suppressed.
• input_range.pdf: This indicates the input current amplitude which makes the circuit saturated when a sinusoidal wave at the amplitude and frequency. For example, 120mA at DC makes the circuit saturated. This is because the first (and second) stage opamp only has the ±12V output range in LISO. This limit is hit when 120mA is given to the transimpedance resistance of 1K.

### Notes on the model

• In the LISO model, LT1028 was used instead of LT1128. However, this is not a problem in our application.
• The output of the circuit is to be differential. This requires a proper noise estimation, strictly to say. i.e., the noises from the output stage are uncorrelated; however, the noise from the previous stages are coherently added. To deal with this issue, I have added a low noise buffer+differential amp using ideal opamps (op00) to the output stage. So notice that this is not a part of the real circuit.

### How to run the LISO model

• The primary model is "D060572.fil" .
• The liso directory contains "fil" which is a Mac version of LISO. Replace this with an appropriate version if you are not on Mac.
• Use a Perl script "mfil" to run different model runs in a batch.
• mfil produces "runAa" files. "A" ranges from 1 (rt=10Ohm) to 6 (rt=3000Ohm).
• "a" ranges from 1 to 3. 1 is the transimpedance gain calculation. 2 is the input noise analysis. 3 is the max range calculation.
• A Matlab script "D060572.m" summarizes the results. It uses my class file "freq_data_tools.m" , which needs to be placed in the path.

KAGRA/Subgroups/IOO/OMC/DCPDCalculation (last edited 2018-10-13 00:57:57 by YutaMichimura)