DMM-16R-AT User Manual V1.31
www.diamondsystems.com
Page 27
7.
ANALOG INPUT CIRCUIT DESCRIPTION
7.1 Resolution
DMM-16R-AT uses a 16-bit A/D converter. This means that the analog input voltage can be measured to the
precision of a 16-bit binary number. The maximum value of a 16-bit binary number is 2
16
- 1, or 65535, so the
full range of numerical values provided by a 16-bit A/D converter is 0 - 65535.
The smallest change in input voltage that can be detected is 1/(2
16
), or 1/65536, of the full-scale input range.
This smallest change results in an increase or decrease of 1 in the A/D code, and so this change is referred to
as 1 LSB, or 1 least significant bit.
7.2 Unipolar and Bipolar Inputs
Diamond-MM-16R-AT can measure both unipolar (positive only) and bipolar (positive and negative) analog
voltages. In general you should select the highest gain (smallest input range) that will allow the A/D converter
to read the full range of voltages for your input signals. However, if you pick too high a gain, then the A/D
converter will clip at either the high end or low end, and you will not be able to read the full range of input
voltages.
7.3 Single Ended and Differential Inputs
Diamond-MM-16R-AT can handle both single-ended and differential inputs. A single-ended input is a two-wire
input (one input signal and ground) that is referenced to analog ground on the board. This means that the
input voltage will be measured with respect to the board’s analog ground. A differential input is a three-wire
input (input +, input -, and ground), and the board will measure the difference between the voltages of the two
inputs. Polarity is important for a differential input. Diamond-MM-16R-AT will subtract the voltage on the low (-
) input from the voltage of the high (+) input. Differential inputs are frequently used when the grounds of the
input device and the measurement device (Diamond-MM-16R-AT) are at different voltages, or when a low-
level signal is being measured that has its own ground wire. Differential inputs also provide better noise
immunity than single-ended inputs, because most of the noise will be present in equal amounts on both the +
and
– inputs, so in the subtraction process the noise will be cancelled out.
7.4 Input Ranges and Resolution
Range
ADBU
G1
G0
Input Range
Resolution (1 LSB)
0
0
0
0
5V
153 V
0
0
0
1
2.5V
76 V
0
0
1
0
1.25V
38 V
0
0
1
1
0.625V
19 V
0
1
0
0
Invalid setting
0
1
0
1
Invalid setting
0
1
1
0
Invalid setting
0
1
1
1
Invalid setting
1
0
0
0
10V
305 V
1
0
0
1
5V
153 V
1
0
1
0
2.5V
76 V
1
0
1
1
1.25V
38 V
1
1
0
0
0
– 10V
153 V
1
1
0
1
0
– 5V
76 V
1
1
1
0
0
– 2.5V
38 V
1
1
1
1
0
– 1.25V
19 V
7.5 Analog Input Wiring
All analog input signals should use one of the analog ground pins on the I/O connector (Agnd) as their
common reference. Connecting the
– input of the analog signal to digital ground will induce significant noise in
the measurement and lower the measurement accuracy to approximately 8-10 bits. For lowest noise, use
shielded wiring when possible, keep the wiring as short as possible, and avoid running analog signals in
parallel with noisy power and digital signals or directly over noisy power and digital circuits. Sharing one Agnd
pin among several analog inputs is acceptable and will not degrade measurements.