2

0

−4

−6

SLEWING10-VPULSE

t − Time − ns

0

50

100

150

200

−2

8

6

4

−8

250

SR=1400V/ms

VCC(H) = ± 15V

VCC(L) = ± 5V

Gain=+5

RF = 1.1kΩ

RL = 25 Ω

TR/TF =1ns

25

75

125

175

225

4

0

−8

−12

SLEWING20-VPULSE

t − Time − ns

0

50

100

150

200

−4

16

12

8

−16

250

SR=4000V/ms

VCC(H) = ± 15V

VCC(L) = ± 5V

Gain=+5

RF = 1.1kΩ

RL = 25 Ω

TR/TF =1ns

25

75

125

175

225

THS6032

www.ti.com....................................................................................................................................................... SLOS233F – APRIL 1999 – REVISED AUGUST 2009

SLEW RATE

The slew rate performance of a current-feedback amplifier like the THS6032 is affected by many different factors.

Some of these factors are external to the device, such as amplifier configuration and PCB parasitics, and others

are internal to the device, such as available currents and node capacitance. Understanding some of these factors

should help the PCB designer arrive at a more optimum circuit with fewer problems.

Whether the THS6032 is used in an inverting amplifier configuration or a noninverting configuration can impact

the output slew rate. Slew rate performance in the inverting configuration is generally faster than the noninverting

configuration. This is because in the inverting configuration the input terminals of the amplifier are at a virtual

ground and do not significantly change voltage as the input changes. Consequently, the time to charge any

capacitance on these input nodes is less than for the noninverting configuration, where the input nodes actually

do change in voltage an amount equal to the size of the input step. In addition, any PCB parasitic capacitance on

the input nodes further degrades the slew rate, simply because there is more capacitance to charge. If the main

supply voltage VCC(H) to the amplifier is reduced, slew rate decreases because there is less current available

within the amplifier to charge the capacitance on the input nodes as well as other internal nodes. Also, as the

load resistance decreases, the slew rate typically decreases due to the increasing internal currents, which slow

down the transitions.

Internally, the THS6032 has other factors that impact the slew rate. The amplifier's behavior during the slew rate

transition varies slightly depending upon the rise time of the input. This is because of the way the input stage

handles faster and faster input edges. Slew rates (as measured at the amplifier output) of less than about 1200

V/

μs are processed by the input stage in a very linear fashion. Consequently, the output waveform smoothly

transitions between initial and final voltage levels. For slew rates greater than 1200 V/

μs, additional

slew-enhancing transistors present in the input stage (transistors Q5 and Q6 in Figure 44) begin to turn on to

support these faster signals. The result is an amplifier with extremely fast slew rate capabilities. The additional

aberrations present in the output waveform with these faster slewing input signals are due to the brief saturation

of the internal current mirrors. This phenomenon, which typically lasts less than 20 ns, is considered normal

operation and is not detrimental to the device in any way. If for any reason this type of response is not desired,

then increasing the feedback resistor or slowing down the input signal slew rate reduces the effect.

Figure 47.

Figure 48.

Copyright © 1999–2009, Texas Instruments Incorporated

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