04-12-2012, 12:38 PM
Precision, Low Power INSTRUMENTATION AMPLIFIERS
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DESCRIPTION
The INA128 and INA129 are low power, general
purpose instrumentation amplifiers offering excellent
accuracy. Their versatile 3-op amp design and small
size make them ideal for a wide range of applications.
Current-feedback input circuitry provides wide bandwidth
even at high gain (200kHz at G = 100).
A single external resistor sets any gain from 1 to
10,000. INA128 provides an industry standard gain
equation; INA129’s gain equation is compatible with
the AD620.
The INA128/INA129 is laser trimmed for very low
offset voltage (50mV), drift (0.5mV/°C) and high common-
mode rejection (120dB at G ³ 100). It operates
with power supplies as low as ±2.25V, and quiescent
current is only 700mA—ideal for battery operated
systems. Internal input protection can withstand up to
±40V without damage.
The INA128/INA129 is available in 8-pin plastic
DIP, and SO-8 surface-mount packages, specified for
the –40°C to +85°C temperature range. The INA128
is also available in dual configuration, the INA2128.
APPLICATION INFORMATION
Figure 1 shows the basic connections required for operation
of the INA128/INA129. Applications with noisy or high
impedance power supplies may require decoupling capacitors
close to the device pins as shown.
The output is referred to the output reference (Ref) terminal
which is normally grounded. This must be a low-impedance
connection to assure good common-mode rejection. A resistance
of 8W in series with the Ref pin will cause a typical
device to degrade to approximately 80dB CMR (G = 1).
DYNAMIC PERFORMANCE
The typical performance curve “Gain vs Frequency” shows
that, despite its low quiescent current, the INA128/INA129
achieves wide bandwidth, even at high gain. This is due to
the current-feedback topology of the input stage circuitry.
Settling time also remains excellent at high gain.
NOISE PERFORMANCE
The INA128/INA129 provides very low noise in most applications.
Low frequency noise is approximately 0.2mVp-p
measured from 0.1 to 10Hz (G ³ 100). This provides
dramatically improved noise when compared to state-of-theart
chopper-stabilized amplifiers.
OFFSET TRIMMING
The INA128/INA129 is laser trimmed for low offset voltage
and offset voltage drift. Most applications require no external
offset adjustment. Figure 2 shows an optional circuit for
trimming the output offset voltage. The voltage applied to
Ref terminal is summed with the output. The op amp buffer
provides low impedance at the Ref terminal to preserve good
common-mode rejection.
INPUT BIAS CURRENT RETURN PATH
The input impedance of the INA128/INA129 is extremely
high—approximately 1010W. However, a path must be provided
for the input bias current of both inputs. This input
bias current is approximately ±2nA. High input impedance
means that this input bias current changes very little with
varying input voltage.
Input circuitry must provide a path for this input bias current
for proper operation. Figure 3 shows various provisions for
an input bias current path. Without a bias current path, the
inputs will float to a potential which exceeds the commonmode
range, and the input amplifiers will saturate.
If the differential source resistance is low, the bias current
return path can be connected to one input (see the thermocouple
example in Figure 3). With higher source impedance,
using two equal resistors provides a balanced input with
possible advantages of lower input offset voltage due to bias
current and better high-frequency common-mode rejection.
LOW VOLTAGE OPERATION
The INA128/INA129 can be operated on power supplies as
low as ±2.25V. Performance remains excellent with power
supplies ranging from ±2.25V to ±18V. Most parameters
vary only slightly throughout this supply voltage range—see
typical performance curves. Operation at very low supply
voltage requires careful attention to assure that the input
voltages remain within their linear range. Voltage swing
requirements of internal nodes limit the input commonmode
range with low power supply voltage. Typical performance
curves, “Input Common-Mode Range vs Output
Voltage” show the range of linear operation for ±15V, ±5V,
and ±2.5V supplies.