+

-

AMPLIFIER UNDER

TEST

+6.5V

-6.5V

RLOAD

1k:

50:

2V

0

0

-2V

OR

HP 8082

PULSE

GENERATOR

TEK 7633

STORAGE SCOPE

+

TEK 7S14

SAMPLING INPUT

VOUT

t

VF

0

TSR

TSET

TPD

LM6584

www.ti.com

SNOSA44C – MAY 2003 – REVISED MARCH 2013

Another important specification for Gamma Buffers is small signal bandwidth and slew rate. When column drivers

select which voltage levels are written to a row of pixels, their internal DACs inject current spikes into the Gamma

Lines. This generates voltage transients at the Gamma Buffer outputs, and they should settle-out in less than

1

μs to insure a steady output voltage from the column drivers. Typically, these transients have a maximum

amplitude of 2V, so a gamma buffer must have sufficient bandwidth and slew rate to recover from a 2V transient

in 1

μs or less.

Figure 35. Large Signal Transient Response of an Operational Amplifier

Figure 35 illustrates how an op amp responds to a large-signal transient. When such a transient occurs at t = 0,

the output does not start changing until TPD, which is the op amp’s propagation delay time (typically 20ns for the

LM6584). The output then changes at the op amp’s slew rate from t = TPD to TSR. From t = TSR to TSET, the

output settles to its final value (VF) at a speed determined by the op amp’s small-signal frequency response.

Although propagation delay and slew limited response time (t = 0 to TSR) can be calculated from data sheet

specifications, the small signal settling time (TSR to TSET) cannot. This is because an op amp’s gain vs. frequency

has multiple poles, and as a result, small-signal settling time can not be calculated as a simple function of the op

amp’s gain bandwidth. Therefore, the only accurate method for determining op amp settling time is to measure it

directly.

Figure 36. Gamma Buffer Settling Time Test Circuit

The test circuit in Figure 36 was used to measure LM6584 settling time for a 2V pulse and 1k

Ω load, which

represents the maximum transient amplitude and output load for a gamma buffer. With this test system, the

LM6584 settled to within ±30mV of 2V pulse in approximately 170ns. Settling time for a 0 to –2V pulse was

slightly less, 150ns. These values are much smaller than the desired response time of 1

μs, so the LM6584 has

sufficient bandwidth and slew rate for regulating gamma line transients.

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