December 2005

Performance of Precision Floating Gate Analog Voltage References at
Cryogenic Temperatures

Richard Patterson, NASA Glenn Research Center

Ahmad Hammoud, QSS Group, Inc. / NASA GRC

**Background**

Precision voltage references based on Intersil's proprietary Floating
Gate Analog (FGA)^TM^ technology are reported to feature very low
temperature coefficient, excellent long term stability, and high
accuracy \[1--2\]. They consume very little power, exhibit very low
drift and noise, and provide excellent line and load regulation. Their
extremely ultra-low supply currents allow these devices to be powered-on
continuously as they present almost no load to the power source,
especially in battery-powered circuits. In addition, reference devices
achieve their highest accuracy and lowest drift when powered up
continuously \[2\]. They can source and sink currents up to about 10 mA
and are specified for operation from --40 °C to +125 °C \[1-2\]. The
performance of two versions of these FGA voltage reference chips at
cryogenic temperatures was investigated in this work.

**Test Procedure**

Two series of Intersil's precision FGA^TM^ voltage reference chips were
mounted on printed circuit boards for evaluation in the temperature
range between +20 °C to --195 °C. These devices, which comprised of
Intersil ISL60002BI and X60008B, were characterized at test temperatures
of 20, -50, -100, -150 and ‑195 °C in a liquid nitrogen cooled
environmental chamber. Each device performance was evaluated in terms of
its output voltage and supply current at different input voltage levels
as a function of temperature. Line regulation was also established at
four load levels and at different temperatures. Finally, the effect of
temperature on load regulation of these devices was also determined.
Table I lists some of the manufacturer's device specifications \[1-2\].

Table I. Manufacturer's specifications of FGA^TM^ voltage reference
chips.

  ------------------------------------- ---------------- ----------------
  **Parameter (Unit)**                  **ISL60002BI**   **X60008B**

  Output Voltage Temp. Coeff., TC       20               3
  V~OUT~ (ppm/°C)                                        

  Supply Current, I~S~ (nA)             400              500

  Source & Sink Current, I~O~ (mA)      7                10

  Supply Voltage Range, V~S~ (V)        2.7 -- 5.5       4.5 - 6.5

  Output Voltage, V~O~ (V)              2.5              2.5

  Temperature Range, T (°C)             -40 - +85        -40 - +85

  Package                               8 lead SOIC      8 lead SOIC

  Lot \#                                MZ41301A         MZ41406A
  ------------------------------------- ---------------- ----------------

**Results and Discussion**

*Output Voltage*

Figure 1 shows the variation in the output voltage of the ISL60002
device as a function of temperature. The data was obtained at four
different input voltages under no load conditions. It can be clearly
seen that, regardless of the input voltage level, the output voltage of
this reference exhibited a gradual but very minute drop with decrease in
temperature. At an input voltage of 4 V for example, the output value
had a value of 2.5003 V at room temperature but it decreased to 2.4996 V
at -195 °C temperature. This corresponds to an output voltage
temperature coefficient of about 3.25 ppm/°C. Similar behavior was
obtained for the X60008 device in terms of its output variation with
temperature, as depicted in Figure 2.

![](media/image1.wmf)

Figure 1. Output voltage of ISL60002 reference as a function of
temperature.

![](media/image2.wmf)

Figure 2. Output voltage of X60008 reference as a function of
temperature.

*Supply Current*

The supply current of the ISL60002 voltage reference is shown in Figure
3 as a function of temperature and at various input voltages. It can be
seen that the supply current, in general, remained almost steady with
change in temperature. With an applied input voltage between 5 and 6
volts, the supply current, however, displayed little higher values;
notably at room temperature. The input voltage range for this reference
is specified between 2.7 V to 5.5 V, and driving it near or beyond the
maximum input voltage is most likely what contributed to the increase in
the supply current.

![](media/image3.wmf)

Figure 3. Supply current of ISL60002 voltage reference versus
temperature.

![](media/image4.wmf)

Figure 4. Supply current of X60008 voltage reference versus temperature.

The supply current of the X60008 voltage reference exhibited excellent
stability with temperature as shown in Figure 4. Regardless of the input
voltage applied, the current did not change at all within the test
temperature of +20 °C to --195 °C.

*Line Regulation*

Line regulation characteristics of the ISL60002 voltage reference was
found not to be influenced much by the change in test temperature
between +20 °C to --195 °C. These curves, which were obtained at four
load levels, are depicted in Figures 5 and 6 at the test temperature of
+20 °C and --195 °C, respectively. The two set of curves were very
similar in their trend as they maintained almost the same change in the
output voltage, ∆ V~OUT~, under similar operating conditions.

![](media/image5.wmf)

Fig 5. Line regulation of ISL60002 at +20 °C.

![](media/image6.wmf)

Fig 6. Line regulation of ISL60002 at -195 °C.

The X60008 voltage reference exhibited similar behavior in line
regulation as that experienced by the ISL60002 reference. The line
regulation curves of the X60008 voltage reference are depicted at +20 °C
and -195 °C in Figures 7 and 8, respectively.

![](media/image7.wmf)

Fig 7. Line regulation of X60008 at +20 °C.

![](media/image8.wmf)

Fig 8. Line regulation of X60008 at -195 °C.

*Load Regulation*

Figure 9 shows the variation in the output voltage of the ISL60002
device as a function of load current at three different temperatures. It
can be seen that the output voltage dropped very slightly as load
current was increased. Varying the test temperature between +20 °C and
-195 °C seemed to produce very little change in the load regulation of
this voltage reference, as displayed in Figure 9.

![](media/image9.wmf)

Figure 9. Load regulation of ISL60002 reference at various temperatures.

Similar behavior was observed in the load regulation characteristics of
the X60008 voltage reference as that displayed by its ISL60002
counterpart. Load regulation curves of this voltage reference are shown
in Figure 10 for test temperatures of +20 °C, -100 °C, and -195 °C.

![](media/image10.wmf)

Figure 10. Load regulation of X60008 reference at various temperatures.

**Conclusions**

Preliminary evaluation was carried out on the performance of precision
voltage reference integrated circuits at cryogenic temperatures.
Intersil's ISL60002BI and X60008B precision voltage reference chips,
which are based on Floating Gate Analog (FGA^TM^) technology, were
tested in the temperature range between +20 °C to --195 °C. Device
performance was obtained in terms of output voltage and supply current
at different input voltage levels as a function of temperature. Line and
load regulation characteristics were also established at four load
levels and at different temperatures. While the output voltage of either
reference dropped slightly with decrease in temperature, the supply
current exhibited minimal change with temperature. Line and load
regulation of both devices were very similar and did not show very
strong dependence on temperature. Although the two references are
specified for operation from -40 °C to +85 °C, they sustained very good
operation at cryogenic temperatures down to -- 195 °C. Therefore, these
devices represent good candidates for potential use in extreme low
temperature applications. Output accuracy and stability under long-term
exposure to extreme temperatures and thermal cycling, however, need to
be established so that reliability assessment of such devices can be
determined for use in space exploration missions.

**References**

1.  Intersil ISL60002 Precision 1.25 & 2.5 Low Voltage FGA^TM^
    References Data Sheet, FN8082.2, September 17, 2004.

2.  Intersil X60008B-25 Precision 2.5 FGA^TM^ Voltage Reference Data
    Sheet, FN8140.0, March 15, 2005.

**Acknowledgments**

This work was performed under the NASA Glenn Research Center GESS
Contract \# NAS3-00145. Funding was provided from the NASA Electronic
Parts and Packaging (NEPP) Program Task "Mixed Signal Devices for
Low/High Temperatures".