Datasheet

1 s/divm
SignalInput
toINP1
Ch4,2V/div
1.5k ConnectingtoINP1W
Ch2,2V/div
DiffSignalOutput
Ch1,1V/div
PGA280
SBOS487A –JUNE 2009–REVISED SEPTEMBER 2009................................................................................................................................................ www.ti.com
Programmable gain amplifiers such as the PGA280 use internal resistors to set the gain. Consequently,
quiescent current is increased by the current that passes through these resistors. The largest amplitude could
increase the supply current by ±0.4mA. In maximum overload, gain of 128V/V and each or the inputs connected
to the opposite supply voltage, a current of approximately 27mA was measured. External resistors in series with
the input pins that are normally present avoid this extreme condition. This current is only limited by the internal
600Ω and the switch-on resistance (see Figure 44).
Current Buffer
Designed for highest accuracy and low noise, both amplifier inputs are protected from dynamic overvoltages
through clamps. The amplifier fast input slew rate (approximately 1V/µs) normally prevents these clamps from
turning on, provided adequate signal filtering is placed before the input. However, the fast channel
switching-transient of a multiplexer or switch is much steeper, and cannot be filtered; this type of transient
generates a dynamic overload. The current buffers (BUF) prevent this dynamic overload condition of the input.
With the buffers not activated, Figure 45 indicates the clamp current flowing as a result of a fast signal change.
The ramp in the signal, measured at the input pins (INP1), is the resulting voltage drop across the 1.5kΩ resistor.
In the example measurement, this resistor is placed between the signal generator and the input pin of the
PGA280.
Figure 45. Buffer OFF: Input Clamp Current Flowing
Figure 46 shows a typical block diagram for multiplexed data acquisition. The transient from channel 1 to channel
2 , shown as a voltage step, dynamically overloads the amplifier. A current pulse results from the input protection
clamp. Without the activation of the buffers (see BUF, Figure 44), the clamp current charges the filter and the
signal source. Input low-pass filters are often set to settling times in the millisecond range; therefore, discharge
currents from dynamic overload would produce long settling delays.
20 Submit Documentation Feedback Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): PGA280