t − Time − ns

SR = 1500 V/

µs

Gain = 5

10

0

5

−5

2

0

0

60

40

20

80

100

140

120

160

180 200

4

−10

−15

t − Time − ns

SR = 2400 V/

µs

Gain = 5

−2

0

5

−5

2

0

0

60

40

20

80

100

140

120

160

180 200

4

−10

−15

THS3001

SLEW RATE

The slew rate performance of a current-feedback amplifier, like the THS3001, 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 THS3001 is used in an inverting amplifier configuration or a noninverting configuration can impact

the output slew rate. As can be seen from the specification tables as well as some of the figures in this data

sheet, slew-rate performance in the inverting configuration is faster than in 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

degrades the slew rate further simply because there is more capacitance to charge. Also, if the supply voltage

to charge the capacitance on the input nodes as well as other internal nodes.

Internally, the THS3001 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

1500 V/

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

transitions between initial and final voltage levels. This is shown in Figure 50. For slew rates greater than 1500

V/

μs, additional slew-enhancing transistors present in the input stage begin to turn on to support these faster

signals. The result is an amplifier with extremely fast slew-rate capabilities. Figure 50 and Figure 51 show

waveforms for these faster slew rates. 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.

SLEW RATE

SLEW RATE

Figure 50.

Figure 51.

Copyright © 1998–2009, Texas Instruments Incorporated

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