Bias Card low noise bias lines noise analysis

The low noise detector bias lines in Rev F bias cards are driven by a bipolar DAC (MAX5444AEUB+) whose output is buffered by an opamp in a non-inverting configuration (AD797), and inverted by a second opamp (AD797). These two signals are then each fed through a series resistance before going to the backplane and MDM connectors. The noise performance of these bias lines is determined by summing the noise contributions of the DAC output resistance, the feedback resistances in the buffer and the voltage and current noise of the op amp. At high frequencies (ignoring 1/f noise), the total noise can be calculated as:

<math> v_{nt,1kHz,300K} = \sqrt{ e_n^2 + i_n R_i + i_n (R_f_1 // R_f_2) + 4kTR_i + 4kTR_f_1(\frac{R_f_2}{R_f_1 + R_f_2})^2 + 4kTR_f2 (\frac{R_f_1}{R_f_1 + R_f_2})^2 } \cong 45 \ \frac{nV}{\sqrt{Hz}} </math>