Differential Mode

ANALOG INPUT

following tables for differ ential inputs.

When channel = odd

Pol.	-	+	-	+	-	+	-	+
CH #	0	1	2	3	4	5	6	7
channel	1			3	5			7
When channel =		even
Pol.	+	-	+	-	+	-	+	-
CH #	0	1	2	3	4	5	6	7
channel	0			2	4			6

When range = 0, the input is ±2.5 v olts and a 1 = 0 to 5 volts.

When d ifferential m ode is specified , inpu ts are actu ally pseudo-differential. What this means is that a ground reference is needed. For example, you cannot place a battery be tween ch annel 0 and 1 and get an ac curate reading. T he (-) input must be referenced to ground. An example of where pseudo-differential works is an output from a bridge network.

Apseudo-differential input subtracts the DC component from an input. T he IC maker recomm ends the (-) input remain stable within 1 count with respect to ground for best results. Connecting a 0. 1 uF capacitor from the (-) input to grou nd wor ks well.

When operating in differential mode, r elative + and - voltages must be connected to specific inputs. When inputs are reversed, a conversion returns a 0. When the relative voltage changes, perform a conversion on the alternate c hannel. CON FIG AIN is p erfor med on b oth channels.

Pairs of channels c an be differ ential while oth ers single ended. Thus, if channel 0 and 1 are differ ential inputs, channels 2-7 may be single ended.

Examples u sing CON FIG AIN

Below are sample syntaxes for CO NFIG AIN

Differential, 0 to + 5V input

CON FIG AIN 0,0, 1

CON FIG AIN 1,0, 1

SECTION 10

Per form a conver sion as nor mal:

A = AIN(0)

The difference between channel 0 and 1 is returned. When channel 1 is more positive than channel 0, the result is zero. The differenc e is read on channel 1 by performing:

A = AIN(1)

Single-ended, ±2.5V input

CONFIG AIN channel, 1 , 0

The result is 0 for -2.500V input, 2048 for 0. 000V, and 4095 for + 2.4988V.

Acquiring Analog Data

Analog data is accessed with the AIN function. The syntax is:

A = AIN(channel)

This function assigns the analog value of a channel to the variable ; A in th is case. The value retur ned is alway s in the 0 to 4095 range bec ause the converter is 12 bits.

Power up or reset default configures inputs to the 0-5V range, single ended.

To view the result of a conversion in the command mode, type:

print ain(0)

The result at channel 0 is returned. T he returned value will always be in the 0 to 4095 range. When using a channel in the ±2. 5V ra nge, the value r eturned is interpreted differently. Zero count is now -2.500V, 4095 is + 4.9988, and 2048 is 0.000V.

Use the following formulas to convert a returned number to a voltage:

0 - 5V A = . 001221 * AIN(channel)

±2.5V A = .001221 * ain( channel) - 2.5

The AIN function require s about 1.5 ms to convert the data. Additional time is needed to store the data. The example below takes 255 data samples and stores them

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