Design Considerations for Medical Applications
One of the fastest evolving markets in the industrial space is medical instrumentation.
Part of the reason of its growth is the high demand from consumers wanting more easyto-use devices at their fingertips. As consumer demand increases, manufacturers of such
devices often look for innovative ways and solutions to optimize their designs for
performance, cost and portability. In portable applications such as pulse oximetry,
electrocardiography and glucose measurements, noise and power consumption are very
important parameters.
Noise is important because there are mandates and worldwide standards which designers
must abide by and a low-noise floor enables a wider dynamic range and hence, better
overall accuracy. Power consumption on the other hand, is a function of current, which is
inversely proportional to voltage noise. Noise is also impacted by the geometry of the
transistors. That is, a low noise IC requires larger transistors, which increase the die size
and make it difficult to fit into small packages.
Given the many aspects of noise, the one that is the most challenging to understand is
burst noise, commonly called popcorn noise. It’s challenging because generally, it isn’t
tested for and is a complex phenomenon. Burst noise can still be encountered today in
some industry standard op amps and is caused by transistors jumping erratically between
2 values of beta (hfe), which cause a step function voltage change at the output of an op
amp.
Ideally, to get a sense of the accuracy of the system, one would add distortion to the noise
calculation, be it relating to the op amp or the data converter. So instead of SNR alone,
we would use SNR + distortion or SINAD as it’s sometimes called. However, when you
combine the noise of the op amp and that of the data converter, you’ll notice that the total
noise has a logarithmic function and is written as 20 log10 (10-SNRAMP/10+10-SNRADC/10)1/2,
which means that the distortion figure is relatively insignificant. In addition, not every op
amp datasheet specifies distortion (THD+N). Suffice it to say that by calculating the total
SNR while omitting distortion generally yields a good approximation.
Some applications like pulse oximetry and hemoglobin measurements are more sensitive
to current noise. Some people tend to assume too quickly that a low input bias current
gives the lowest possible current noise. However, it can be the opposite with op amps
using input bias cancellation schemes.
Chopper amps as they’re commonly called, offer the advantage of eliminating flicker
noise (1/f) and offer superb precision, but because of the output ripple they tend to
exhibit, they generally require anti-aliasing filters when interfacing with high resolution
ADCs. The choice of filters depends greatly on the application, but common ones are the
Butterworth, which is optimal with respect to passband ripple and the Bessel also known
as Thomson, which provides a constant group delay (linear phase).
Another factor that affects the dynamic range is the dropout voltage of the amplifier.
Even in the presence of a light resistive load, the output voltage never gets to zero. The
output has an inherent limitation regardless of the process –it’s limited by either the
saturation voltage (VCE) in the case of a bipolar output or by the Rdson as in the case of a
CMOS output stage. For example, with National Semiconductor’s micro power
LMP2231 configured in a single supply operation and a load of 10 kΩ, it has a dropout
voltage of 50 mV. This means that with a full scale voltage of 0V to 5V for a 12-bit
ADC, there will be a loss of 40 bit counts according to the equation 50E - 3/(5/2^12).
Likewise, for a 16-bit ADC under the same conditions, there will be 655 bit counts lost
out of the total 65,536. To remedy this situation, use National’s LM7705 negative bias
generator with virtually any single supply amplifier. Note however, that careful
consideration to noise should be taken when using higher resolution ADCs and precision
voltage references.