DESCRIPCIÓN GENERAL

The AD8236 is the lowest power instrumentation amplifier in the industry. It has rail-to-rail outputs and can operate on voltages as low as 1.8 V. Its 40 μA maximum supply current makes it an excellent choice in battery-powered applications.
The AD8236’s high input impedance, low input bias current of 1 pA, high CMRR of 110 dB (G = 100), small size, and low power offer tremendous value. It has a wider common-mode voltage range than typical three-op-amp instrumentation amplifiers, making this a great solution for applications that operate on a single 1.8 V or 3 V supply. An innovative input stage allows for a wide rail-to-rail input voltage range without the crossover distortion common in other designs.
The AD8236 is available in an 8-lead MSOP and is specified over the industrial temperature range of −40°C to +125°C.

 

CARACTERÍSTICAS

Low power: 40 μA supply current (maximum)
Bajas corrientes de entrada
1 pA input bias current
0.5 pA input offset current
High CMRR: 110 dB CMRR, G = 100
Space-saving MSOP
Zero input crossover distortion
Rail-to-rail input and output
Gain set with single resistor
Operates from 1.8 V to 5.5 V

 

APLICACIONES

Instrumentación médica
Low-side current sense
Portable devices

 

TEORÍA DE FUNCIONAMIENTO

The AD8236 is a monolithic, 2-op-amp instrumentation amplifier. It was designed for low power, portable applications where size and low quiescent current are paramount. For example, it has a rail-to-rail input and output stage to offer more dynamic range when operating on low voltage batteries. Unlike traditional rail-to-rail input amplifiers that use a complementary differential pair stage and suffer from nonlinearity, the AD8236 uses a novel architecture to internally boost the supply rail, allowing the amplifier to operate rail to rail yet still deliver a low 0.5 ppm of nonlinearity. In addition, the 2-op-amp instrumentation amplifier architecture offers a wide operational common-mode voltage range. Additional information is provided in the Common Mode Input Voltage Range section. Precision, laser-trimmed resistors provide the AD8236 with a high CMRR of 86 dB (minimum) at G = 5 and gain accuracy of 0.05% (maximum).

 

LAYOUT

Careful board layout maximizes system performance. In applications that need to take advantage of the low input bias current of the AD8236, avoid placing metal under the input path to minimize leakage current.

 

INPUT BIAS CURRENT RETURN PATH

The AD8236 input bias current is extremely small at less than 10 pA. Nonetheless, the input bias current must have a return path to common. When the source, such as a transformer, cannot provide a return current path, one should be created

 

INPUT PROTECTION

All terminals of the AD8236 are protected against ESD. In addition, the input structure allows for dc overload conditions a diode drop above the positive supply and a diode drop below the negative supply. Voltages beyond a diode drop of the supplies cause the ESD diodes to conduct and enable current to flow through the diode. Therefore, an external resistor should be used in series with each of the inputs to limit current for voltages above +VS. In either scenario, the AD8236 safely handles a continuous 6 mA current at room temperature.
For applications where the AD8236 encounters extreme overload voltages, as in cardiac defibrillators, external series resistors and low leakage diode clamps, such as BAV199Ls, FJH1100s, or SP720s, should be used.

 

LOW POWER HEART RATE MONITOR

The low power and small size of the AD8236 make it an excellent choice for heart rate monitors. The AD8236 measures the biopotential signals from the body. It rejects common-mode signals and serves as the primary gain stage set at G = 5. The 4.7 μF capacitor and the 100 kΩ resistor set the −3 dB cutoff of the high-pass filter that follows the instrumentation amplifier. It rejects any differential dc offsets that may develop from the half-cell overpotential of the electrode.