dataTaker - Data Loggers, Powerful and Flexible Data Acquisition & Data Logging Systems

Measuring Low Level Voltages

Measuring low level voltages is the simplest measurement to make with the dataTaker.

This section describes the measurement of low level differential (double ended) DC voltages and single ended DC voltages, in the range of approximately –3250 mV to +3250 mV.

DataTaker data loggers which have solid state (CMOS) and relay multiplexers are able to measure low level voltages in this range. In the case of loggers with the relay multiplexer, the low level voltage ranges do not select the internal attenuators.

Discussion throughout the remainder of this chapter refers to all models of the dataTaker, and is not specific between loggers with solid state (CMOS) or relay multiplexers.

The dataTaker 50 has 5 differential analog input channels, or 10 single ended input channels, which can be used for low level voltage measurements.

The dataTaker 500/600 series loggers and the Channel Expansion Module (CEM-AD) have 10 differential analog input channels, or 30 single ended input channels, which can be used for low level voltage measurements.

Any combination of these differential and single ended connections can be used for measuring low level voltages.

Low level voltage signals can be measured in a number of ways as follows

as differential inputs on any of the analog input channels.

as single ended inputs referenced to Analog Return.

as single ended inputs referenced to an external common.

All dataTaker data loggers have three low level voltage measurement ranges as follows

±25.000 mV full scale

±250.00 mV full scale

±2500.0 mV or ±2.500 Volt full scale

The highest range provides for approximately 30% over-ranging, allowing voltages of ñ3250.0 mV to +3250.0 mV to be measured.

These voltage measuring ranges are selected automatically by the dataTaker to provide the best measurement resolution for a given voltage. Measurement ranges do not have to be selected by the user.

The Channel Expansion Module (CEM-AD) measures voltage in the same ranges as the dataTaker to which it is connected.

The dataTaker 505, dataTaker 515, dataTaker 605 and dataTaker 615 data loggers also support 3 high level voltage ranges (See Section II ñ Measuring High Level Voltages).

Differential Measurement of Low Level Voltage

In the differential mode, the signal voltage is applied between the +ve and the ñve terminals of the analog input channels. The differential voltage measured by the dataTaker is the difference of the voltages between the +ve and ñve terminals.

The differential mode of voltage measurement is used for

Floating Sources - when neither of the outputs from the signal source can be connected to ground.

Grounded Sources - when one of the outputs from the signal source is grounded independently of the dataTaker. Connecting it to the dataTaker ground could result in a ground loop.

Noisy Signals - when noise is equally present in both sides of the sensor output (e.g. mains or line hum). Differential mode largely cancels out this common mode noise.

Common Mode Voltage Range 

The voltage between either of the +ve or the ñve analog input terminals of the analog input channel and dataTaker ground must not exceed ±3.5 Volts (dataTaker 50, 500, 600) or ±100 Volts (dataTaker 505, 515, 605, 615). This is the maximum allowable common mode voltage for the analog inputs.

Sensors which have no direct connection to ground can be tied to the dataTaker ground, via the internal 1 MOhm input termination resistors. When analog channels are used to measure voltage, the channels default to this configuration.

However if there is high common mode noise due to capacitive pickup, these 1 MOhm terminating resistors may not be sufficient to maintain the common mode range.

Under these conditions an additional resistor of 10 KOhm or less must be placed between one input line and the Analog Return terminal or any GND terminal. This is illustrated in Figure 35 below

 

 

Figure 35 ñ Grounding of Floating Sensors

 

When grounding sensors to the dataTaker ground, the Ground (GND) terminals can be used either directly or via a suitable resistor.

The dataTaker Analog Return (R) terminals can also be used to ground sensors. However if Analog Return terminals are used to ground sensors, the internal 100 Ohm current shunt between the Analog Return terminal and dataTaker ground will be in series with the external grounding resistor (see Figure 44).

Differential Measurement

Low level voltages in the range of ñ2500 mV to +2500 mV can be connected directly to the analog channels of the dataTaker data loggers for differential measurement, as follows

 

 

Figure 36 ñ Connecting Low Level Voltages as Differential Inputs

 

The differential voltage measurement is made between the +ve and ñve terminals of the analog input channel. The dataTaker also measures the polarity of the input voltage.

Low level voltages connected to the dataTaker data loggers as differential inputs are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, differential low level voltage signals are measured by the commands for example

BEGIN
 RA10M
  1V  5V  10V
END

which instructs the dataTaker to measure the differential voltages applied between the +ve and ñve terminals of analog channel 1, 5 and 10. The V indicates that the signal connected to these channels is to be measured as a low level voltage. The data is returned in units of mV.

Using DeLogger, differential low level voltage signals can be measured by the following Program Builder program. The differential connections are selected from the Voltage Wiring Configurations dialog which opens when you have selected the analog input channel.

 

 

By default the 1 MOhm termination resistors will be switched in during measurement of the differential low level voltages.

The dataTaker will read the voltage inputs every 10 minutes, and readings are stopped by a H (Halt) command.

Single Ended Measurement of Low Level Voltage

Low level signal voltages can also be measured by the dataTaker data loggers as single ended voltages in two ways, which differ in their reference configuration as follows

as single ended inputs to the +ve, ñve or T terminals, referenced to Analog Return

as single ended inputs to the +ve, ñve or T terminals, referenced to an external common point connected to the SE REF terminal

The dataTaker treats the voltage signal applied to the +ve, ñve or S input terminal of the analog input channel as the active input, and measures this with respect to Analog Return or SE REF.

The dataTaker 50 does not support single ended voltage measurement for the Excite (T) terminal.

Single Ended Measurement Referenced to Analog Return

Low level voltages in the range  ñ2500 mV to +2500 mV can be connected to the analog channels for measurement as single ended voltages referenced to Analog Return

 

 

Figure 37 ñ Connecting Low Level Voltages as Single Ended Inputs
Referenced to Analog Return

 

The single ended voltage is measured between the +ve, ñve or T terminal and the Analog Return terminal of the analog input channel. The dataTaker also measures polarity of the input voltage. In the single ended input mode, the 1 MOhm input termination resistors are not switched in during measurement.

Low level voltages connected as single ended inputs and referenced to Analog Return are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, single ended low level voltages are measured by the command

BEGIN
 RA5M
  1+V  5 TV  8-V
END

which instructs the dataTaker to measure the single ended voltages as follows

between the +ve terminal of channel 1 (1+) and the Analog Return terminal

between the T terminal of channel 5 (5 T and the Analog Return terminal

between the ñ terminal of channel 8 (8ñ) and the Analog Return terminal

The Analog Return terminal of all analog channels is connected internally to dataTaker ground. The V indicates that the signal applied to these channels is to be measured as a voltage. The data is returned in units of mV.

Using DeLogger, single ended low level voltage signals are measured by the following Program Builder program. The single ended connections are selected from the Voltage Wiring Configurations dialog which opens when you have selected the analog input channel.

 

The dataTaker will read the voltage inputs every 5 minutes, and readings are stopped by a H (Halt) command.

Single Ended Measurement Referenced to an External Common

Low level voltages in the range of ñ2500 mV to +2500 mV can be connected directly to the analog channels of the dataTaker for measurement as single ended voltages which are referenced to an external common, as illustrated in Figure 38 overleaf.

 

 

Figure 38 ñ Connecting Low Level Voltages as Single Ended Inputs
Referenced to an External Common

The single ended voltage is measured between the +ve, ñve or T terminal of the analog channel, and the SE REF terminal. Low level voltages connected as single ended inputs referenced to an external common are sampled and the data is returned when a Schedule containing the channel is executed.

Using DeTransfer, single ended low level voltage signals are measured with respect to the common SE REF terminal by the command for example

BEGIN
 RA5M
  1+V(X)  5 TV(X)  8-V(X)
END

which instructs the dataTaker to measure the single ended voltages as follows

between the +ve terminal of analog input channel 1 (1+) and an external reference point connected to the SE REF terminal

between the T terminal of analog input channel 5 (5 T) and an external reference point connected to the SE REF terminal

between the ñve terminal of analog input channel 8 (8ñ) and an external reference point connected to the SE REF terminal

The V indicates that the signal applied to the channels is to be measured as a voltage. The X channel option indicates that the voltage is to measured with respect to an external reference connected to the SE REF input. The data is returned in units of mV.

Using DeLogger, single ended low level voltage signals are measured by the following Program Builder program. The single ended connections referenced to SE REF terminal are selected from the Voltage Wiring Configurations dialog which opens when you have selected the analog input channel.

 

The dataTaker will read the inputs every 5 minutes, and readings are stopped by entering a H (Halt) command.

Measurement Ranges and Accuracy

All dataTaker models have three low level voltage measurement ranges of ±25.000 mV, ±250.00 mV and ±2500.0 mV full scale.

The upper range provides approximately 30% over-ranging, which extends this measurement range to ±3250 mV.

These measurement ranges are selected automatically by the dataTaker, to provide the best measurement resolution for a given input voltage.

The low level voltage measurement accuracy is typically better than 0.10%, and can be trimmed via software to greater accuracy by calibrating the dataTaker against a standard reference voltage (See Section III ñ Trimming for Errors in the Internal Reference Voltage).

For differential inputs, the measurement range is determined by the difference of the voltages at the +ve and ñve terminals of the analog input channels.

The absolute value of voltage at the input terminals does not affect the measurement range or accuracy, except for common mode voltage effects.

The common mode rejection ratio of >90 db means that the common mode voltage at the input terminals will have an effect in the input voltage of less than 0.001% of the common mode voltage.

The following table summarizes the measurement ranges, resolution and accuracies for low level voltage inputs.

 

Input Voltage Range

Resolution

Measurement
Accuracy *

<±25.00mV

1mV

±0.10%

±250.0mV

10mV

±0.10%

±2500.0mV

100mV

±0.02%

 

   *  Measurement accuracy can be trimmed

Error Messages

Input voltage signals which fall outside the range the common mode range of the dataTaker will produce an over-range reading of ñ99999.9 mV or +99999.9 mV respectively.

The dataTaker also reports the out of range error condition with the error message ‘E11ñinput(s) out of rangeí if the Messages Switch /M is enabled.

Page Content


Home

Title and Waranty

Go to: Section 2 | Section 3

Section 1


Construction of the dataTaker 50

Construction of the dataTaker 500 600

Construction of the CEM

Getting Started

 

Section 2


Interfacing

Powering the dataTaker

Powering Sensors from the dataTaker

The Serial Interfaces

The RS232 COMMS Serial Interface

The NETWORK Interface

Analog Process

Connect Analog

Analog Chns

Measuring Low Level Voltages

Measuring High Level Voltages

Measuring Currents

Measuring 4-20mA Current Loops

Measuring Resistance

Measuring Frequency and Period

Measuring Analog Logic State

Measuring Temperature

Measuring Temperature with Thermocouples

Measuring Temperature with RTDs

Measuring Temperature with IC Temperature Sensors

Measuring Temperature with Thermistors

Measuring Bridges and Strain Gauges

Measuring Vibrating Wire Strain Gauges

The Digital Input Channels

Monitoring Digital State

The Low Speed Counters

The Phase Encoder Counter

The High Speed Counters

The Digital Output Channels

The Channel Expansion Module

Installing The Panel Mount Display

 

Section 3


Programming the dataTaker

Communication Protocols and Commands

Entering Commands and Programs

Format of Returned Data

Specifying Channels

The Analog Input Channels

The Digital Input Channels

The Counter Channels

The Digital Output Channels

The Real Time Clock

The Internal Channels

Channel Options

Schedules

Alarms

Scaling Data - Polynomials, Spans and Functions

CVs Calcs and Histogram

Logging Data to Memory

Programming from Memory Cards

STATUS RESET TEST

Switches and Parameters

Networking

Writing Programs

Keypad and Display

Error Mess Text

Appendix A - ASCII

Appendix B - ADC Timing