MEDICAL INDUSTRY FOCUS
HOW HIGH RESOLUTION voltage sources can benefit the medical industry
Ultrahigh precision programmable voltage sources are proving of benefit for the medical industry by, for example, giving radiologists the clarity and resolution they need to see smaller anatomical structures. Michael Lynch, Analog Devices, comments
T
he AD5791 with the LTZ1000, ADA4077 and AD8675/AD8676 can be
used to provide a programmable voltage source that achieves 1ppm resolution with 1ppm INL, and better than 1ppm FSR long- term drift. Within the medical industry, this will help radiologists with superior image clarity, resolution and contrast. Applied to an MRI (magnetic resonance imaging), this advanced imagery of organs and soft tissues will allow medical professionals to more accurately detect heart problems, tumours, cysts and abnormalities in various parts of the body. For test and measurement systems, the
1ppm resolution and accuracy improves overall test equipment accuracy and granularity, leading to finer control and excitation of external sources and nano-actuators. In industrial automation, this resolution and accuracy provide the precision that is required to move, alter or position an actuator on the nanoscale.
THE AD5791 The AD5791 is a 20-bit, unbuffered voltage output digital to analog converter with 1ppm relative accuracy (1LSB INL) and 1LSB DNL (guaranteed monotonic). It has a 0.05ppm/˚C temperature drift, 0.1ppm p-p noise, and better than 1ppm long-term stability. Containing a precision R-2R architecture that exploits state-of-the-art, thin film resistor matching techniques, this operates from a bipolar supply up to 33V, and it can be driven by a positive reference in the range of 5V to VDD -2.5V and a negative reference in the range of VSS 2.5V to 0V. The AD5791 uses a versatile 3-wire serial interface that operates at clock rates up to 35MHz and is compatible with standard SPI, QSPI, MICROWIRE, and DSP interface standards. The AD5791 is offered in a 20-lead TSSOP package. The LTZ1000 is an ultrastable
temperature controllable reference. It provides 7.2V output with a 1.2µV p-p of
significantly add to the total noise level. The reference buffers used to drive the
REFP and REFN pins of the AD5791 must be configured in unity gain. Any extra currents flowing through a gain setting resistor into the reference sense pins will degrade the accuracy of the DAC. The AD5791 INL performance is
marginally sensitive to the input bias current of the amplifiers used as reference buffers. For this reason, amplifiers with low input bias currents were chosen.
noise, long-term stability of 2μV/√kHr, and temperature drifts of 0.05ppm/˚C. The part contains a buried Zener reference, a heater resistor for temperature stabilisation, and a temperature sensing transistor. External components are used to set operating currents and to temperature stabilise the reference – this allows for maximum flexibility and the best long-term stability and noise. The ADA4077 is a high precision,
low noise operational amplifier with a combination of extremely low offset voltage and very low input bias currents. Unlike JFET amplifiers, the low bias and offset currents are relatively insensitive to ambient temperatures, even up to 125˚C. Outputs are stable with capacitive loads of more than 1000pF with no external compensation. The AD8675/AD8676 are
precision, rail-to-rail operational amplifiers with ultralow offset, drift, and voltage noise combined with very low input bias currents over the full operating temperature range.
Programmable voltage source
DRIFT To maintain a low temperature drift coefficient for the entire system, the individual components chosen must have low temperature drift. The AD5791 has a TC of 0.05ppm FSR/˚C, the LTZ1000 offers a TC of 0.05ppm/˚C, and the ADA4077 and the AD8675 contribute 0.005ppm FSR/˚C and 0.01ppm FSR/˚C, respectively. Long-term drift is another important
parameter that can cause significant accuracy limitations in systems. Long- term stability for the AD5791 is typically better than 0.1ppm/1000 hours at 125˚C. Long-term stability on the order of 1μV per month can be achieved with the LTZ1000.
“Applied to an MRI, this advanced imagery of
organs and soft tissues will allow medical professionals to more accurately detect heart problems...”
CIRCUIT CONSIDERATIONS Noise: Low frequency noise must be kept to a minimum to avoid impact on the DC performance of the circuit. In the 0.1Hz to 10Hz bandwidth, the AD5791 generates about 0.6μV p-p noise, each ADA4077 will generate 0.25μV p-p noise, the AD8675 will generate 0.1μV p-p noise, and the LTZ1000 generates 1.2μV p-p noise. Resistor values were chosen to ensure that their Johnson noise will not
/ DESIGNSOLUTIONS
LAB RESULTS INL error was measured at ambient temperature in the lab by varying the input code to the AD5791 from zero-scale to full-scale with a code step of five. The voltage at the output of the output buffer (AD8675) was recorded at each code using an 8.5 digit DVM. The results were well within the ±1LSB specification. The noise measured at
mid-scale was 1.1μV p-p and the noise measured at full scale was 3.7μV p-p. The noise contribution
from each voltage reference path is attenuated by the DAC when mid-scale code is selected—hence the lower noise
figure for mid-scale code. The system long-term drift was
measured at 25˚C. The AD5791 was programmed to 5V (¾ scale) and the output voltage was measured every 30 minutes over a period of 1000 hours. Drift values less than 1ppm FSR were observed.
Analog Devices
www.analog.com DESIGN SOLUTIONS | SEPTEMBER 2018 33
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