ADS-951 查看數據表（PDF） - DATEL Data Acquisition products

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ADS-951

18-Bit, 1MHz, Low-Power Sampling A/D Converters

DATEL Data Acquisition products  

ADS-951

18-Bit, 1MHz, Low-Power Sampling A/D Converters

TECHNICAL NOTES

1. Obtaining fully specified performance from the ADS-951 requires careful

attention to pc-card layout and power supply decoupling. The device's ana-

log and digital ground systems are not connected to each other internally.

For optimal performance, tie all ground pins (2, 4, 7, 28, and 31) directly to

a large analog ground plane beneath the package.

Bypass all power supplies to ground with 10μF tantalum capacitors in par-

allel with 0.1μF ceramic capacitors. Locate the bypass capacitors as close

to the unit as possible. The +4.22V REFERENCE OUTPUT (pin 1) should not

be bypassed externally. If needed to drive external circuitry, pin 1 should be

buffered close to the unit.

2. The ADS-951 achieves its specified accuracies without the need for exter-

nal calibration. If required, the device's small initial errors can be reduced

to zero using the adjustment circuitry shown in Figure 2. When using this

circuitry, or any similar offset and gain calibration hardware, make adjust-

ments following warmup. To avoid interaction, always adjust offset before

gain. Connect pins 5 and 6 to ground if not using offset and gain adjust

circuits.

3. Applying a start convert pulse while a conversion is in progress (EOC =

logic "1") will initiate a new and probably inaccurate conversion cycle. Data

for the interrupted and subsequent conversions will be invalid.

THERMAL REQUIREMENTS

All DATEL sampling A/D converters are fully characterized and tested over

operating temperature (case) ranges of 0 to +70°C, –40 to +110°C, and

–55 to +125°C. All room-temperature (TA = +25°C) production testing

is performed without the use of heat sinks or forced-air cooling. Thermal

impedance figures for each device are listed in their respective specifica-

tion tables.

These devices do not normally require heat sinks, however, standard pre-

cautionary design and layout procedures should be used to ensure devices

do not overheat. The ground and power planes beneath the package, as

well as all pcb signal runs to and from the device, should be as heavy

as possible to help conduct heat away from the package. Electrically-

insulating, thermally-conductive "pads" may be installed underneath the

package. Devices should be soldered to boards rather than "socketed", and

of course, minimal air flow over the surface can greatly help reduce the

package temperature.

CALIBRATION PROCEDURE

Any offset/gain calibration procedures should not be implemented until the

device is fully warmed up. To avoid interaction, adjust offset before gain.

The ranges of adjustment for the circuits in Figure 2 are guaranteed to

compensate for the ADS-951's initial accuracy errors and may not be able

to compensate for additional system errors. If Gain and Offset adjustment

pins are not used they should be left floating.

A/D converters are calibrated by positioning their digital outputs exactly

on the transition point between two adjacent digital output codes. This is

accomplished by connecting LED's to the digital outputs and performing

adjustments until certain LED's "flicker" equally between on and off. Other

approaches employ digital comparators or microcontrollers to detect when

the outputs change from one code to the next.

For the ADS-951, offset adjusting is normally accomplished when the

analog input is 0 minus ½LSB (–19μV). See Table 2 for the proper bipolar

output coding.

Gain adjusting is accomplished when the analog input is at nominal full

scale minus 1½LSB's (–4.999943V).

Zero/Offset Adjust Procedure

1. Apply a train of pulses to the START CONVERT input (pin 27) so that the

converter is continuously converting.

2. For bipolar zero/offset adjust, apply –19μV to the ANALOG INPUT (pin 3).

3. Adjust the offset potentiometer until the output code flickers equally be-

tween 01 1111 1111 1111 1111 and 10 0000 0000 0000 0000.

Gain Adjust Procedure

1. Apply –4.999943V to the ANALOG INPUT (pin 3).

2. Adjust the gain potentiometer until all output bits are 1's and the LSB flick-

ers between 1 and 0.

3. To confirm proper operation of the device, vary the applied input voltage to

obtain the output coding listed in Table 2.

INPUT VOLTAGE ZERO ADJUST GAIN ADJUST

RANGE

(–½ LSB) (–FS +1½ LSB)

±5V

–19μV

–4.999943

Table 1. Input Connections

BIPOLAR

SCALE

+FS –1 LSB

+3/4 FS

+1/2 FS

0

–1/2 FS

–3/4 FS

–FS +1 LSB

–FS

COMPLEMENTARY OFFSET BINARY

INPUT RANGE

OUTPUT CODING

±5V

MSB

LSB

+4.999962

00 0000 0000 0000 0000

+3.750000

00 0111 1111 1111 1111

+2.500000

00 1111 1111 1111 1111

+0.000000

01 1111 1111 1111 1111

–2.500000

10 1111 1111 1111 1111

–3.750000

11 0111 1111 1111 1111

–4.999962

11 1111 1111 1111 1110

–5.000000

11 1111 1111 1111 1111

Table 2. Output Coding

DATEL, Inc. 11 Cabot Boulevard, Mansfield, MA 02048-1151 USA • Tel: (508) 339-3000 • www.datel.com • e-mail: help@datel.com

10 Sep 2015 MDA_ADS-951.E01 Page 4 of 7

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