Low Temperature Testing of the LTC1419 A/D Converter

Scott Gerber & Ahmad Hammoud

Dynacs Engineering

Richard Patterson

NASA Glenn Research Center

Functional testing of the LTC1419 A/D Converter was performed over the
temperature range of 25C to ‑190C. The Linear Technology LTC1419 (1419)
is a 14-bit, 800ksps sampling A/D converter. Samples of two different
grades of the LTC1419 devices were obtained both in the (SW) package.
The first was the commercial (C) grade, with a manufacturer's specified
operating temperature range of 0C to 70C. The second was the (AI)
version (better linearity) in the industrial grade, with a
manufacturer's specified operating temperature range of -40C to 85C. A
test circuit was constructed such that a logic analyzer and pattern
generator could be interfaced to the 1419. Figure 1 shows the test
setup. The test setup was configured such that the 1419 was operated in
mode 1a as defined in the Linear Technology device data sheet. In this
mode, the /CS and /RD pins are tied low, thus selecting the 1419 and
always enabling the data outputs. A falling edge on the /CONVST pin will
start a conversion. A rising edge on the /BUSY pin signals that the
output data can be latched.

**Test Results 1419C (Commercial Grade)**

The 1419 device can be operated using its own 2.5V internal reference or
by an external user specified reference (by overdriving the Vref pin).
The internal reference was selected for initial testing of the 1419
device. Testing of the 1419C was initiated at room temperature (22C).
Testing involved recording the /CONVST and /BUSY signals on a digital
oscilloscope, monitoring the reference voltage on a digital multimeter,
and recording the data outputs with a logic analyzer for several dc
input values to the 1419C analog inputs. The dc inputs spanned the
(+2.5V to -2.5V) range of the 1419C with a 2.5V reference. The analog
input range is dependent on the reference voltage level. Table 1 shows
the results of testing the 1419C with the internal reference selected.
The logic analyzer recorded the digital output of the 1419C and
displayed the corresponding decimal equivalent. This decimal equivalent
when multiplied by the equivalent volts/bit gives the measured voltage
of the data output. As seen in Table 1, output data was obtained down to
-50C. As the temperature was decreased to -75C the internal reference
voltage dropped from 2.492 volts to 0.906 volts (somewhere around -60C).
At this point, the device has become effectively non-functional, that is
there is no reference to measure the inputs by. At this point, a 2.5
volt external reference was applied to the Vref input. Surprisingly, the
2.5V external reference did not overdrive the internal reference at
temperatures below -60C. However, if the external reference was reduced
to 1.25 Volts (the minimum specified external reference voltage), the
1419C would operate. Table 2 and 3 show the results of the 1419C testing
with a 1.25 volt external reference. With the 1.25V external reference
the 1419C device worked quite well down to -190C. However, some
noticeable increase in measured offset between the input and measured
output voltage was observed (Table 3).

![](media/image1.wmf)

Table 1. Measured Output Voltage vs. Analog Input Voltage at various

temperatures (1419C with 2.5V internal reference).

  ----------- ----------- ----------- ----------- ----------- -----------
              Measured                                        
              output (V)                                      

  Ain (V)     22C         0C          -25C        -50C        -60C

  2.5         2.496       2.495       2.495       2.492       

  2           2.003       2.003       2.003       2.000       

  1.5         1.502       1.501       1.504       1.502       

  1           1.002       1.003       1.003       1.002       

  0           0.001       0.001       0.001       0.001       

  -1          -1.001      -1.001      -1.001      -1.000      

  -1.5        -1.503      -1.502      -1.504      -1.502      

  -2          -2.002      -2.001      -1.996      -1.995      

  -2.5        -2.496      -2.495      -2.495      -2.492      

  **Vref**    **2.496**   **2.495**   **2.495**   **2.492**   **0.906**
  ----------- ----------- ----------- ----------- ----------- -----------

Table 2. Measured Output Voltage vs. Analog Input Voltage at

various temperatures (1419 with 1.25V external reference).

+-------+---------+---------+---------+---------+---------+---------+
|       | M       |         |         |         |         |         |
|       | easured |         |         |         |         |         |
|       | Output  |         |         |         |         |         |
|       | (V)     |         |         |         |         |         |
+-------+---------+---------+---------+---------+---------+---------+
| A     | -75C    | -100C   | -125C   | -150C   | -175C   | -190C   |
| nalog |         |         |         |         |         |         |
|       |         |         |         |         |         |         |
| Input |         |         |         |         |         |         |
| (V)   |         |         |         |         |         |         |
+-------+---------+---------+---------+---------+---------+---------+
| 1     | 0.9953  | 0.9953  | 0.9950  | 0.9829  | 0.9806  | 0.9760  |
+-------+---------+---------+---------+---------+---------+---------+
| 0.5   | 0.4967  | 0.4964  | 0.4926  | 0.4917  | 0.4877  | 0.4869  |
+-------+---------+---------+---------+---------+---------+---------+
| 0     | 0.0001  | 0       | 0       | 0       | 0       | 0       |
+-------+---------+---------+---------+---------+---------+---------+
| -0.5  | -0.5001 | -0.5003 | -0.4975 | -0.4950 | -0.4918 | -0.4883 |
+-------+---------+---------+---------+---------+---------+---------+
| -1    | -0.9976 | -0.9970 | -0.9927 | -0.9904 | -0.9821 | -0.9772 |
+-------+---------+---------+---------+---------+---------+---------+

Table 3. Measured Offset with 1.25V external reference (1419C).

+-------+---------+---------+---------+---------+---------+---------+
|       | M       |         |         |         |         |         |
|       | easured |         |         |         |         |         |
|       | Offset  |         |         |         |         |         |
|       | (Analog |         |         |         |         |         |
|       | Input-M |         |         |         |         |         |
|       | easured |         |         |         |         |         |
|       | Output) |         |         |         |         |         |
|       | (V)     |         |         |         |         |         |
+-------+---------+---------+---------+---------+---------+---------+
| A     | -75C    | -100C   | -125C   | -150C   | -175C   | -190C   |
| nalog |         |         |         |         |         |         |
|       |         |         |         |         |         |         |
| Input |         |         |         |         |         |         |
| (V)   |         |         |         |         |         |         |
+-------+---------+---------+---------+---------+---------+---------+
| 1     | 0.0047  | 0.0047  | 0.0050  | 0.0171  | 0.0194  | 0.0240  |
+-------+---------+---------+---------+---------+---------+---------+
| 0.5   | 0.0033  | 0.0036  | 0.0074  | 0.0083  | 0.0123  | 0.0130  |
+-------+---------+---------+---------+---------+---------+---------+
| 0     | -0.0002 | 0       | 0       | 0       | 0       | 0       |
+-------+---------+---------+---------+---------+---------+---------+
| -0.5  | 0.0001  | 0.0003  | -0.0025 | -0.0050 | -0.0082 | -0.0117 |
+-------+---------+---------+---------+---------+---------+---------+
| -1    | -0.0024 | -0.0030 | -0.0073 | -0.0096 | -0.0179 | -0.0228 |
+-------+---------+---------+---------+---------+---------+---------+

Figures 2 shows the /CONVST input signal and the 1419C /BUSY output
signal at 25C and --175C, respectively. Figure 2 also shows the
conversion time (t~conv~) as defined by the manufacturer's data sheet.
The manufacturer's data sheet specifies the typical and maximum
conversion times to be 950ns and 1150ns, respectively. Table 4 shows the
measured conversion times for the 1419C from 25C to -190C.

# ![](media/image2.wmf){width="4.186111111111111in" height="6.786111111111111in"}

# Figure 2. The /CONVST input signal and the /BUSY output signal at 25C and --175C, respectively. 

Table 4. LTC1419 Conversion Time vs Temperature

  ------------- ------ ------ ------ ------ ------ ------- ------- ------- ------- -------
  Temperature   22C    0C     -25C   -50C   -75C   -100C   -125C   -150C   -175C   -190C

  t~conv~ (ns)  920    920    920    880    872    856     840     840     840     840
  ------------- ------ ------ ------ ------ ------ ------- ------- ------- ------- -------

**[Test Results LTC1419AISW (Industrial Grade)]{.underline}**

The internal reference was selected for initial testing of the 1419AI
device. Testing of the 1419AI was initiated at room temperature (22C)
with the same setup used previously. Table 5 and 6 show the results of
testing the 1419AI with the internal reference selected. As seen in
Table 5, output data was obtained down to -50C. As the temperature was
decreased to -75C the reference voltage dropped from 2.49 volts to 0.937
volts (somewhere around -72C). This is the same behavior observed for
the commercial grade 1419. At this point, the device has become
effectively non-functional, that is there is no reference to measure the
inputs by. At this point, a 2.5 volt external reference was applied to
the Vref input. As before, the 2.5V external reference did not overdrive
the internal reference at temperatures below -60C. However, if the
external reference was reduced to 1.25 Volts (the minimum specified
external reference voltage), the 1419 would operate. We were able to
increase the external reference voltage to 1.6V and still have the
1419AI operate. Table 7 and 8 show the results of the 1419AI testing
with a 1.6 volt external reference. With the 1.6V external reference the
1419AI device worked quite well down to -190C. However, some noticeable
increase in measured offset between the input and measured output
voltage was observed (Table 8).

Table 5. Measured Output Voltage vs. Analog Input Voltage at various

temperatures (LTC1419AISW with 2.5V internal reference).

+---------+---------+---------+---------+---------+---------+---------+
|         | M       |         |         |         |         |         |
|         | easured |         |         |         |         |         |
|         | output  |         |         |         |         |         |
|         | (V)     |         |         |         |         |         |
+---------+---------+---------+---------+---------+---------+---------+
| Analog  | 22C     | 0C      | -25C    | -50C    | -72C    | \*\*25C |
|         |         |         |         |         |         |         |
| Input   |         |         |         |         |         |         |
| (V)     |         |         |         |         |         |         |
+---------+---------+---------+---------+---------+---------+---------+
| 2.5     | 2.498   | 2.496   | 2.493   | 2.489   |         | 2.497   |
+---------+---------+---------+---------+---------+---------+---------+
| 2       | 1.999   | 1.998   | 1.994   | 1.993   |         | 1.997   |
+---------+---------+---------+---------+---------+---------+---------+
| 1.5     | 1.500   | 1.499   | 1.497   | 1.496   |         | 1.497   |
+---------+---------+---------+---------+---------+---------+---------+
| 1       | 1.000   | 0.999   | 0.998   | 0.997   |         | 1.000   |
+---------+---------+---------+---------+---------+---------+---------+
| 0       | 0.001   | 0.001   | -0.000  | 0       |         | 0       |
+---------+---------+---------+---------+---------+---------+---------+
| -1      | -1.000  | -1.000  | -1.000  | -0.997  |         | -1.000  |
+---------+---------+---------+---------+---------+---------+---------+
| -1.5    | -1.5    | -1.498  | -1.502  | -1.496  |         | -1.499  |
+---------+---------+---------+---------+---------+---------+---------+
| -2      | -1.998  | -1.997  | -1.996  | -1.991  |         | -1.997  |
+---------+---------+---------+---------+---------+---------+---------+
| -2.5    | -2.498  | -2.496  | -2.493  | -2.49   |         | -2.497  |
+---------+---------+---------+---------+---------+---------+---------+
| *       | **      | **      | **      | *       | **      | **      |
| *Vref** | 2.498** | 2.496** | 2.493** | *2.49** | 0.937** | 2.497** |
+---------+---------+---------+---------+---------+---------+---------+

Table 6. Measured Offset (LTC1419AISW with 2.5V internal reference).

+----------+----------+----------+----------+-----------+----------+
|          | Measured |          |          |           |          |
|          | Offset   |          |          |           |          |
|          | (Analog  |          |          |           |          |
|          | Input-   |          |          |           |          |
|          | Measured |          |          |           |          |
|          | Output)  |          |          |           |          |
|          | (V)      |          |          |           |          |
+----------+----------+----------+----------+-----------+----------+
| Analog   | 22C      | 0C       | -25C     | -50C      | \*\*25C  |
|          |          |          |          |           |          |
| Input    |          |          |          |           |          |
| (V)      |          |          |          |           |          |
+----------+----------+----------+----------+-----------+----------+
| 2.5      | -0.002   | -0.004   | -0.007   | -0.011    | -0.003   |
+----------+----------+----------+----------+-----------+----------+
| 2        | -0.001   | -0.002   | -0.006   | -0.007    | -0.003   |
+----------+----------+----------+----------+-----------+----------+
| 1.5      | 0.000    | -0.001   | -0.003   | -0.004    | -0.003   |
+----------+----------+----------+----------+-----------+----------+
| 1        | 0.000    | -0.001   | -0.002   | -0.003    | -0.000   |
+----------+----------+----------+----------+-----------+----------+
| 0        | 0.001    | 0.001    | -0.000   | 0         | 0        |
+----------+----------+----------+----------+-----------+----------+
| -1       | -0.000   | 0.001    | -0.000   | 0.003     | 0.000    |
+----------+----------+----------+----------+-----------+----------+
| -1.5     | 0.000    | 0.002    | -0.002   | 0.004     | 0.001    |
+----------+----------+----------+----------+-----------+----------+
| -2       | 0.002    | 0.003    | 0.004    | 0.009     | 0.003    |
+----------+----------+----------+----------+-----------+----------+
| -2.5     | 0.002    | 0.004    | 0.007    | 0.01      | 0.003    |
+----------+----------+----------+----------+-----------+----------+

Table 7. Measured Output Voltage vs. Analog Input Voltage at various

temperatures (LTC1419AISW with 1.6V external reference).

+------+--------+--------+--------+--------+--------+--------+-------+
|      | Me     |        |        |        |        |        |       |
|      | asured |        |        |        |        |        |       |
|      | Output |        |        |        |        |        |       |
|      | (V)    |        |        |        |        |        |       |
+------+--------+--------+--------+--------+--------+--------+-------+
| An   | -75C   | -100C  | -125C  | -150   | -175C  | -190C  | \*    |
| alog |        |        |        |        |        |        | \*25C |
|      | 1.25V  | 1.6V   | 1.6V   | 1.6V   | 1.6V   | 1.6V   |       |
| I    | ref    | ref    | ref    | ref    | ref    | ref    | 1.6V  |
| nput |        |        |        |        |        |        | ref   |
| (V)  |        |        |        |        |        |        |       |
+------+--------+--------+--------+--------+--------+--------+-------+
| 1.6  |        | 1.562  | 1.556  | 1.556  | 1.553  | 1.553  | 1.569 |
+------+--------+--------+--------+--------+--------+--------+-------+
| 1.5  |        | 1.469  | 1.456  | 1.456  | 1.455  | 1.455  | 1.481 |
+------+--------+--------+--------+--------+--------+--------+-------+
| 1.25 | 1.221  | 1.231  | 1.219  | 1.212  | 1.211  | 1.211  | 1.235 |
+------+--------+--------+--------+--------+--------+--------+-------+
| 1.0  | 0.981  | 0.980  | 0.969  | 0.969  | 0.967  | 0.967  | 0.987 |
+------+--------+--------+--------+--------+--------+--------+-------+
| 0    | -0.002 | -0.000 | 0      | 0      | -1.6   | 0      | -     |
|      |        |        |        |        |        |        | 0.002 |
+------+--------+--------+--------+--------+--------+--------+-------+
| -1.0 | -0.985 | -0.985 | -0.981 | -0.975 | -0.988 | -0.988 | -     |
|      |        |        |        |        |        |        | 0.987 |
+------+--------+--------+--------+--------+--------+--------+-------+
| -    | -1.230 | -1.23  | -1.225 | -1.224 | -1.225 | -1.225 | -     |
| 1.25 |        |        |        |        |        |        | 1.237 |
+------+--------+--------+--------+--------+--------+--------+-------+
| -1.5 |        | -1.478 | -1.474 | -1.472 | -1.462 | -1.481 | -     |
|      |        |        |        |        |        |        | 1.486 |
+------+--------+--------+--------+--------+--------+--------+-------+
| -1.6 |        | -1.581 | -1.574 | -1.567 | -1.581 | -1.581 | -     |
|      |        |        |        |        |        |        | 1.586 |
+------+--------+--------+--------+--------+--------+--------+-------+

Table 8. Measured Offset with 1.6V external reference (LTC1419AISW).

  ------- ---------------- --------- --------- --------- --------- --------- ---------
          Measured Offset                                                    
          (Analog                                                            
          Input-Measured                                                     
          Output) (V)                                                        

  Ain (V) -75C             -100C     -125C     -150C     -175C     -190C     \*\*25C

  1.6                      -0.038    -0.044    -0.044    -0.047    -0.047    -0.031

  1.5                      -0.031    -0.044    -0.044    -0.045    -0.045    -0.019

  1.25    -0.029           -0.019    -0.031    -0.038    -0.039    -0.039    -0.015

  1       -0.019           -0.020    -0.031    -0.031    -0.033    -0.033    -0.013

  0       -0.002           -0.000    0         0         -1.600    0         -0.002

  -1      0.015            0.016     0.019     0.025     0.0124    0.012     0.013

  -1.25   0.020            0.020     0.025     0.026     0.025     0.025     0.013

  -1.5                     0.022     0.026     0.028     0.038     0.019     0.014

  -1.6                     0.019     0.026     0.033     0.019     0.019     0.014
  ------- ---------------- --------- --------- --------- --------- --------- ---------