ISL21009 Intersil Corporation, ISL21009 Datasheet - Page 10

• Manufacturer Part Number: ISL21009
• Description: Low Noise Voltage References
• Manufacturer: Intersil Corporation

Data Converters in particular can utilized the ISL21009 as an
external voltage reference. Low power DAC and ADC
circuits will realize maximum resolution with lowest noise.
Board Mounting Considerations
For applications requiring the highest accuracy, board
mounting location should be reviewed. The device uses a
plastic SOIC package which will subject the die to mild
stresses when the PC board is heated and cooled and
slightly changes shape. Placing the device in areas subject
to slight twisting can cause degradation of the accuracy of
the reference voltage due to these die stresses. It is normally
best to place the device near the edge of a board, or the
shortest side, as the axis of bending is most limited at 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.
Noise Performance and Reduction
The output noise voltage in a 0.1Hz to 10Hz bandwidth is
typically 4µV
bandpass filter 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 at 12.6Hz to create a filter with a 9.9Hz bandwidth.
Noise in the 10kHz to 1MHz bandwidth is approximately
40µV
measurement is made with a 2 decade bandpass filter made
of a 1 pole high-pass filter with a corner frequency at 1/10 of
the center frequency and 1-pole low-pass filter with a corner
frequency at 10 times the center frequency. Load capacitance
up to 1000pF can be added but will result in only marginal
improvements in output noise and transient response. The
output stage of the ISL21009 is not designed to drive heavily
capactive loads, so for load capacitances above 0.001µF the
noise reduction network shown in Figure 30 is recommended.
This network reduces noise significantly over the full
bandwidth. Noise is reduced to less than 20µV
1MHz using this network with a 0.01µF capacitor and a 2kΩ
resistor in series with a 10µF capacitor. Also, transient
response is improved with higher value output capacitor. The
0.01µF value can be increased for better load transient
response with little sacrifice in output stability.
P-P
with no capacitance on the output. This noise
P-P
. The noise measurement is made with a
10
P-P
from 1Hz to
ISL21009
Turn-On Time
The ISL21009 devices have low supply current and thus the
time to bias up internal circuitry to final values will be longer
than with higher power references. Normal turn-on time is
typically 100µs. This is shown in Figure 11. Circuit design
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
extremes of measurement (T
divided by the nominal reference voltage (at T = +25°C) and
multiplied by 10
specifying temperature coefficient.
Output Voltage Adjustment
The output voltage can be adjusted up or down by 2.5% by
placing a potentiometer from Vout to ground, and connecting
the wiper to the TRIM pin. The TRIM input is high impedance,
so no series resistance is needed. The resistor in the
potentiometer should be a low tempco (<50ppm/°C) and the
resulting voltage divider should have very low tempco
<5ppm/°C. A digital potentiometer such as the ISL95810
provides a low tempco resistance and excellent resistor and
tempco matching for trim applications.
V
IN
= 3.5V
FIGURE 30. HANDLING HIGH LOAD CAPACITANCE
0.1µF
10µF
6
HIGH
to yield ppm/°C. This is the “Box” method for
– V
LOW
ISL21009
GND
HIGH
V
), and divide by the temperature
IN
– T
V
O
LOW
0.01µF
). The result is
February 14, 2007
FN6327.1
2kΩ
10µF