= DC PD for OMC =

 * In-vacuum OMC DC PD Pre-amplifier Board [[https://gwdoc.icrr.u-tokyo.ac.jp/cgi-bin/private/DocDB/ShowDocument?docid=9177|JGW-D1809177]]

== OMC DC PD amplifier analysis ==
The full package for the analysis of OMC DCPD amplifier ([[https://dcc.ligo.org/LIGO-D060572|LIGO-D060572]]) with various transimpedance (Made by Koji Arai)

 * [[attachment:OMC_DCPD_amp_model.zip]]

=== 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.