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ISL21090
Device Operation
Precision Bandgap Reference
The ISL21090 uses a bandgap architecture and special trimming
circuitry to produce a temperature compensated, precision
voltage reference with high input voltage capability and
moderate output current drive. Low noise performance is
achieved using optimized biasing techniques. Key features for
precision low noise portable applications, such as handheld
meters and instruments, are supply current (900μA) and noise
(0.1Hz to 10Hz bandwidth) 1.0μV P-P to 6.2μV P-P . Data Converters
in particular can utilize the ISL21090 as an external voltage
reference. Low power DAC and ADC circuits will realize maximum
resolution with lowest noise. The device maintains output voltage
during conversion cycles with fast response, although it is helpful
to add an output capacitor, typically 1 μ F. In the case of the 1.25V
option, a 0.01μF capacitor must be added to the COMP (pin 3) for
stabilization purposes, and a minimum of 0.1μF capacitor must be
added at the output.
Applications Information
Board Mounting Considerations
For applications requiring the highest accuracy, the board
mounting location should be reviewed. The device uses a plastic
SOIC package, which subjects the die to mild stresses when the
printed circuit (PC) board is heated and cooled, which slightly
changes the shape. Because of these die stresses, placing the
device in areas subject to slight twisting can cause degradation
of reference voltage accuracy. It is normally best to place the
device near the edge of a board, or on the shortest side, because
the axis of bending is most limited in that location. Mounting the
device in a cutout also minimizes flex. Obviously, mounting the
device on flexprint or extremely thin PC material will likewise
cause loss of reference accuracy.
Board Assembly Considerations
Some PC board assembly precautions are necessary. Normal
output voltage shifts of 100μV to 500μV can be expected with
Pb-free reflow profiles or wave solder on multi-layer FR4 PC
boards. Precautions should be taken to avoid excessive heat or
extended exposure to high reflow or wave solder temperatures.
Noise Performance and Reduction
The output noise voltage in a 0.1Hz to 10Hz bandwidth is typically
1.9μV P-P ( V OUT = 2.5V). The noise measurement is made with a
bandpass filter. The filter is made of a 1-pole high-pass filter, with a
corner frequency at 0.1Hz, and a 2-pole low-pass filter, with a
corner frequency (3dB) at 9.9Hz, to create a filter with a 9.9Hz
bandwidth. Noise in the 10Hz to 1kHz bandwidth is approximately
1.6μV RMS (V OUT = 2.5V), with 0.1μF capacitance on the output.
This noise measurement is made with a 2 decade bandpass filter.
The filter is made of a 1-pole high-pass filter with a corner
frequency at 10Hz of the center frequency, and 1-pole low-pass
filter with a corner frequency at 1kHz. Load capacitance up to
10μF can be added but will result in only marginal improvements
in output noise and transient response.
18
Turn-On Time
Normal turn-on time is typically 150μs, as shown in Figure 27.
The circuit designer must take this into account when looking at
power-up delays or sequencing.
Temperature Coefficient
The limits stated for temperature coefficient (Tempco) are governed
by the method of measurement. The overwhelming standard for
specifying the temperature drift of a reference is to measure the
reference voltage at two temperatures, take the total variation,
(V HIGH – V LOW ), and divide by the temperature extremes of
measurement (T HIGH – T LOW ). The result is divided by the nominal
reference voltage (at T = +25°C) and multiplied by 10 6 to yield
ppm/°C. This is the “Box” method for specifying temperature
coefficient.
Output Voltage Adjustment
The output voltage can be adjusted above and below the
factory-calibrated value via the trim terminal. The trim terminal is
the negative feedback divider point of the output op amp. The
positive input of the amplifier is about 1.216V, and in feedback,
so will be the trim voltage. The trim terminal has a 5000 ?
resistor to ground internally, and in the case of the 2.5V output
version, there is a feedback resistor of approximately 5000 ?
from V OUT to trim.
The suggested method to adjust the output is to connect a very
high value external resistor directly to the trim terminal and
connect the other end to the wiper of a potentiometer that has a
much lower total resistance and whose outer terminals connect
to V OUT and ground. If a 1M ? resistor is connected to trim, the
output adjust range will be ±6.3mV. It is important to minimize
the capacitance on the trim terminal to preserve output amplifier
stability. It is also best to connect the series resistor directly to
the trim terminal, to minimize that capacitance and also to
minimize noise injection. Small trim adjustments will not disturb
the factory-set temperature coefficient of the reference, but
trimming near the extreme values can.
FN6993.5
February 27, 2013
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