LTC2995 datasheet(71/90 Pages) AD

71 / 90 page

LTM2985

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

DIRECT ADC MEASUREMENTS

In addition to measuring temperature sensors, the

LTM2985 can perform direct ADC (i.e. voltage) mea-

surements. Any channel may be configured to perform

direct single-ended or differential measurements. Direct

ADC channel assignments follow the general convention

shown in Figure 55. The 32-bit channel assignment word

is programmed into a memory location corresponding to

the input channel.

Figure 55. Direct ADC Channel Assignment Conventions

2985 F55

CHADC

SINGLE-ENDED

CHANNEL

ASSIGNMENT = CHADC (1 ≤ CHADC ≤ 10)

= CHADC (2 ≤ CHADC ≤ 10)

24-BIT

∆∑ ADC

24-BIT

∆∑ ADC

CHANNEL

ASSIGNMENT

DIFFERENTIAL

COM

CHADC

CHADC-1

–

Figure 56. Integral Nonlinearity as a Function of

Temperature at VCC2 = 5.25V

DIFFERENTIAL INPUT VOLTAGE (V)

2985 F37

–5

–10

–15

–20

–1.5

–0.5

0.5

1.5

–1

90°C

25°C

–45°C

The Direct ADC mode is configurable to single-ended as

well as differential inputs. The positive input channel ties

to CHADC for both single-ended and differential modes.

For single-ended measurements the ADC negative input is

COM while for differential measurements it is CHADC-1.For

single-ended measurements, COM can be driven with any

voltage above GND – 50mV and below VCC2 – 0.3V. The

direct ADC results are available in memory at a location

corresponding to the conversion channel.

TherearetworesultmodeoptionsfordirectADCoperation.

The first mode is direct voltage output and the second is

table-driven output. When configured for direct voltage

output, the LTM2985 will return a 24-bit fixed-point volt-

age result along with the 8-bit fault status byte. When

configured for table-lookup, the LTM2985 will perform a

table lookup on the raw ADC voltage and return a 24-bit

signed-integer table-lookup result along with the 8-bit

fault status byte.

Figure 56 shows typical integral nonlinearity variation

as a function of supply voltage and temperature for a

differential input voltage (±VREF/2) and VREF/2 common

mode input voltage.

LTC2995 Datasheet(PDF) 71 Page - Analog Devices

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