8.3.4 Slew Rate and Output Stage

In addition to wide bandwidth, the OPA859-Q1 features a high slew rate of 2750 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-Q1 implies that the device accurately

reproduces a 2-V, sub-ns pulse edge, as seen in Figure 6-20. The wide bandwidth and slew rate of the OPA859-

Q1 make it an excellent amplifier for high-speed signal-chain front ends.

Figure 8-7 shows the open-loop output impedance of the OPA859-Q1 as a function of frequency. To achieve

high slew rates and low output impedance across frequency, the output swing of the OPA859-Q1 is limited to

approximately 3 V. The OPA859-Q1 is typically used in conjunction with high-speed pipeline ADCs and flash

ADCs that have limited input ranges. Therefore, the OPA859-Q1 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

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

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OPA859-Q1

SBOSA59 – FEBRUARY 2021

Copyright © 2021 Texas Instruments Incorporated

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