8.3.4 Slew Rate and Output Stage

In addition to wide bandwidth, the OPA859 features a high slew rate of 1150 V/µs. The slew rate is a

critical parameter in high-speed pulse applications with narrow sub-10-ns pulses, such as optical time-domain

reflectometry (OTDR) and LIDAR. The high slew rate of the OPA859 implies that the device accurately

reproduces a 2-V, sub-ns pulse edge; see also Figure 6-20. The wide bandwidth and slew rate make the

OPA859 an excellent amplifier for high-speed signal-chain front ends.

Figure 8-7 shows the open-loop output impedance of the OPA859 as a function of frequency. To achieve

high slew rates and low output impedance across frequency, the output swing of the OPA859 is limited to

approximately 3 V. The OPA859 is typically used in conjunction with high-speed pipeline ADCs and flash ADCs

that have limited input ranges. Therefore, the OPA859 output swing range coupled with the class-leading voltage

noise specification for a CMOS amplifier maximizes the overall dynamic range of the signal chain.

Frequency (Hz)

0

2

4

6

8

10

12

14

16

18

20

10k

100k

1M

10M

100M

1G

10G

D601

Figure 8-7. Open-Loop Output Impedance (ZOL) vs Frequency

8.3.5 Current Noise

The input impedance of CMOS and JFET input amplifiers at low frequencies exceed several GΩs. However,

at higher frequencies, the transistors parasitic capacitance to the drain, source, and substrate reduces the

impedance. The high impedance at low frequencies eliminates any bias current and the associated shot noise.

At higher frequencies, the input current noise increases (see Figure 8-8) as a result of capacitive coupling

between the CMOS gate oxide and the underlying transistor channel. This phenomenon is a natural artifact of

the construction of the transistor and is unavoidable.

Frequency (Hz)

1k

10k

100k

1M

10M

100M

1G

1p

10p

100p

100f

10f

1f

D607

Figure 8-8. Input Current Noise (IBN and IBI) vs Frequency

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OPA859

SBOS852A – SEPTEMBER 2018 – REVISED MAY 2025

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