Select transimpedance gains to align the
measurement current range within the range of the ADC. For the ADS7066, the input
range is programmed to 5 V. Using this configuration, the peak current range is calculated
by dividing the input range by the feedback resistor, RFB, which yields 25 μA
for a 200‑kΩ resistor and 250 μA for a 20‑kΩ feedback resistor.
The current measurement LSB size is
5 V / (RF × 65536). The result yields 381 pA resolution for a 200‑kΩ feedback
resistor, and 3.81 nA resolution for a 20‑kΩ resistor.
A dc voltage is used on the noninverting pin of
the amplifier for two important reasons. The first reason is to reverse-bias the
photodiode, which helps reduce photodiode capacitance and makes sure the photodiode does
not operate in a forward-bias state. The second reason is to keep the output voltage of
the amplifier from coming too close to the negative supply (V–) voltage when the input
current is zero. If the output voltage comes within approximately 40 mV (assuming a 10‑kΩ
load), the amplifier enters a saturation state, which results in loss of open-loop gain
and slow transient response to exit the state (overload recovery). Typically 100 mV is
enough to make sure that the amplifier does not saturate.
A feedback capacitor can be used to help the
stability of the circuit. Typically, if the feedback capacitor has a higher capacitance
than the total input capacitance, advanced compensation schemes are not necessary to
maintain stability of the amplifier along with the capacitance of the photodiode. This
configuration can limit the usable bandwidth of the circuit; see Section 8.1.3.1 for further details.