ADA4530-1ARZ-R7 datasheet(48/51 Pages) AD

48 / 51 page

Data Sheet

ADA4530-1

Rev. A | Page 47 of 50

The total RTI error over the entire temperature range is less

than 150 fA, which is equal to 300 ppm of the 500 pA full-scale

range. Note that the extraordinarily low input bias current of

the ADA4530-1 is not a significant contributor to the total error

over temperature. The interaction of 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 feed-

back 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.

–120

–100

–80

–60

–40

–20

–12

–10

–8

–6

–4

–2

070

TEMPERATURE (°C)

VSY = 10V

VCM = VSY/2

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,

surface-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 elimi-

nates the frequency peaking completely (blue curve) but it

reduces the −3 dB bandwidth from 390 Hz to 50 Hz.

0.1

100

10k

100k

FREQUENCY (Hz)

VSY = 10V

VCM = VSY/2

TA = 25°C

CF = 0fF

CF = 300fF

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 considerable (20%) overshoot. The compensated circuit

(blue curve) is overdamped.

–140

–120

–100

–80

–60

–40

–20

–14

–12

–10

–8

–6

–4

–2

040

TIME (ms)

VSY = 10V

TA = 25°C

IPD = 10pA

CF = 0fF

CF = 300fF

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

resistance, 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.

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