ADA4530-1ARZ-R7 datasheet(47/52 Pages) AD

47 / 52 page

Data Sheet

ADA4530-1

PHOTODIODE INTERFACE

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Rev. C | 47 of 52

the offset voltage with the shunt resistance of the photodiode is the

most significant error source.

This circuit was constructed as described with a 10 GΩ feedback

resistor (Ohmite RX-1M1008JE). The dc error performance was

measured over the 25°C to 60°C temperature range (see Figure

128). The error increases rapidly with temperature as the shunt

resistance changes the noise gain exponentially. The total RTI error

ranges from +2 fA to −10 fA, considerably lower than the worst

case error budget, as expected.

Figure 128. DC Error vs. Temperature

The ac performance of the circuit was also measured. The circuit

was initially constructed without a physical feedback capacitor as

a baseline. The transimpedance gain vs. frequency is shown in

Figure 129. The 30% frequency peaking seen in the frequency

response (red curve) indicates that the feedback loop is marginally

compensated with parasitic capacitance.

A physical capacitor was added to improve the loop compensa-

tion. This capacitor is a 300 fF C0G ceramic in a Size 0805, sur-

face-mount package (AVX UQCFVA0R3BAT2A\500). C0G ceramic

capacitors are good candidates for electrometer circuits because

they have adequate insulation resistance and dielectric absorption

performance.

These low valued capacitors are designed for RF use and are

readily available. The 300 fF capacitor eliminates the frequency

peaking completely (blue curve) but it reduces the −3 dB bandwidth

from 390 Hz to 50 Hz.

Figure 129. Transimpedance Gain vs. Frequency

The stability improvement can be seen in the time domain as well.

The circuits step response to a 10 pA photocurrent is shown in

Figure 130. The uncompensated circuit (red curve) shows consid-

erable (20%) overshoot. The compensated circuit (blue curve) is

overdamped.

Figure 130. 10 pA Step Response

A noise budget is constructed based on the Noise Analysis section.

The RTO noise budget is separated into noise sources integrated

with a low bandwidth (see Table 14) and those integrated with a

high bandwidth (see Table 15).

The low frequency noise contributors include the feedback resist-

ance, the shunt resistance and the amplifier current noise. Each of

these sources has a −3 dB bandwidth equal to the signal bandwidth

(50 Hz); this is equivalent to a noise bandwidth of 79 Hz. The most

significant noise source is the photodiode shunt resistance by a

large margin. The second most significant source is the feedback

resistor. The amplifier current noise is so low that it can be ignored.

Table 14. Low Frequency Noise Budget

Error Source

25°C

45°C

60°C

VNRF

12.8 µV/√Hz

13.2 µV/√Hz

13.5 µV/√Hz

RSHUNT

5 GΩ

1.25 GΩ

442 MΩ

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