With a 2.7-GHz GBWP, the OPA818 is an excellent choice for the design requirements. A challenge with the

calculated component results is practically realizing a 0.092-pF capacitor. Such a small capacitor is realized by

using a capacitive tee network formed by C1, C2, and CT (see Figure 8-3). The equivalent capacitance, CEQ, of

the tee network is given by Equation 2:

CEQ =  C1 × C2

C1 + C2 + CT

(2)

The tee network forms a capacitive attenuator from input to output with C1 and CT, and from output to input

with C2 and CT. With the value of CT being higher than C1 or C2, only a fraction of the output signal is seen by

C1. This network results in a much smaller shunting current provided to the input through C1, and this reduced

shunting current effect is equivalent to how a much smaller capacitor behaves. At a fixed frequency, a smaller

capacitor has a higher impedance, and thus reduced current. Keep the same level of attenuation from input to

output, and vice versa. To find the appropriate capacitor values for the tee network, chose an arbitrarily low but

practically realizable and equal values for capacitors C1 and C2, set CEQ = CTOT, and use Equation 3 to get the

value of the tunable capacitor, CT. The values of capacitors C1, C2, and CT in Figure 8-3 are determined using

this process.

CEQ = C1 × C2 – C1 + C2 × CEQ 

CEQ

(3)

Figure 8-4 shows the TINA-TI™ simulation software closed-loop bandwidth response of the circuit in Figure

8-3. The circuit is designed for f–3dB = 24 MHz and the simulated closed-loop 3-dB frequency is 24.6 MHz

with approximately 0.1

‑dB peaking. The OPA818 TINA-TI software model models the input common-mode and

differential capacitors that are not added externally when simulating in the TINA-TI software. Figure 8-5 shows

the noise simulation of the TIA circuit. The output-referred voltage noise shows on the Y-axis to the left. The

input-referred current noise, which is essentially output-referred voltage noise divided by the transimpedance

gain of 100k, shows on the secondary Y-axis to the right. The simulation results are fairly accurate because

the OPA818 TINA-TI software model closely models the voltage and current noise performance of the amplifier.

The flat-band output voltage noise is 41 nV/√Hz that is equivalent to 0.41 pA/√Hz of input-referred current noise.

The noise in relatively low frequency region where the noise gain of the amplifier is 1 V/V is dominated by

the thermal noise of the 100

‑kΩ resistor (40.7 nV/√Hz at 27°C). At mid-frequencies beyond the zero formed by

RF and CTOT, the noise gain of the amplifier amplifies the voltage noise of the amplifier. The amplifier noise

starts to become the dominant noise contributor from this frequency onward, before the output noise starts to

roll off at frequencies beyond the 3-dB closed-loop bandwidth. When looking at the integrated root-mean-square

(RMS) noise, mid-frequency noise is potentially a significant contributor. Therefore, use a 2.2-nV/√Hz low-noise

amplifier, such as the OPA818, to minimize total RMS noise in the system.

8.2.1.3 Application Curves

Frequency (Hz)

95

95.5

96

96.5

97

97.5

98

98.5

99

99.5

100

100.5

1M

10M

D103

Figure 8-4. Simulated Closed-Loop Bandwidth of

the TIA

Frequency (Hz)

10

0.1

20

0.2

30

0.3

50

0.5

70

0.7

100

1

200

2

300

3

500

5

700

7

1000

10

10

100

1k

10k

100k

1M

10M

100M

D104

Figure 8-5. Simulated TIA Noise

www.ti.com

OPA818

SBOS940B – MAY 2019 – REVISED DECEMBER 2025

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