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SNLA399 July 2022 OPA607

2.3.2 Maximum Current Calculations

The absolute maximum reading supported by the signal chain is given by the value of current which saturates the output of amplifier to supply voltage. So absolute maximum current (I_abs) is given by,

Equation 7.

I_{a b s} = \frac{V_{C C}}{R_{f}} = \frac{3.3}{60 x 10^{6}} = 55 n A

The AC current reading supported by TIA in such case is,

Equation 8.

I_{i n_m a x} = \frac{V_{C C}}{R_{f}} - I_{o f f s e t}

where I_offsetrefers to the DC offset contributed by the diode dark current, offset voltage, input bias current and any other illumination current even in the absence of particle depending upon the optical chamber construction. For this simulation diode BWP34S was assumed, and it was considered that even with the exponential increase in current with rising temperature, the offset current (I_offset) would be below 1 nA.

Maximum AC current limit is related to ADC by the expression,

Equation 9.

I_{i n_m a x} \leq \frac{V_{r e f}}{R_{f}} = \frac{3}{60 x 10^{6}} = 50 n A

as V_ref is the maximum input voltage which can be converted by the ADC.