Differential Amplifier Module

The differential amplifier module was developed to provide a means of monitoring voltage and current at low frequencies. This turned out to have a number of difficulties that required some careful thought to resolve.

Specification

The module was expected initially to work in a low voltage, high current environment (solar power system). However extension to voltages up to 400V was envisaged.

• As the amplifier ICs generally do not tolerate supply voltages more than a diode drop (0.6V) above the supply voltages, it was decided to add a manual attenuator switch. The attenuation ranges chosen were 1, 3, 10 and 30 to allow a good match between the A/D converter precision and the voltage ranges likely to be encountered. An AC/DC switch was also provided.

• Gains of up to 1000 are envisioned to deal with low voltages used for current measurement. Four gains of 1, 10, 100 and 1000 were selected. These can be provided programmatically, allowing the microprocessor to improve the precision of a measurement dynamically.

• It was decided that a programmed zero level calibration would save significantly on costs of precision components and manual trimming of amplifier offsets. This turned out to be somewhat unnecessary as the zero point is quite stable, however it has other advantages, such as eliminating the need for a manual offset calibration.
  

Component Selection

Choice of components was influenced by the desire to use readily available components that would remain available for some time. However programmable gain differential amplifiers are not particularly common. The following somewhat specialized components were chosen (Analog Devices produce a range of excellent quality signal processing ICs):

• AD625 programmable gain differential amplifier. This provides a very simple means of providing for a range of software selectable gains.

• ADG409 dual 4-input multiplexer. This is used for selecting the resistor network to set the amplifier gain. It was chosen for its low series resistance and small variation between channels.

• AD7512 DPDT electronic switch for automatic zeroing. The functionality of this device matches well the application of switching between an external circuit and a zeroing circuit. It also has an added bonus of having bipolar supplies to allow bipolar signals to be switched, and it has an inbuilt protection circuit that will handle up to 40V overvoltage at the inputs. This saves substantially on the need to provide this circuitry in discrete form.

• LF444 or T034/T064 JFET operational amplifiers for high impedance buffering the inputs of the amplifier.

Design

The design difficulties focus around the need for the module to be electrically isolated from the system under test. For systems that are not isolated, problems arise due to the common ground point between the measurement system and the circuit being measured. This can result in currents flowing that will disturb the measurement, or worse. Isolating the module electrically becomes very complex given that the module must interface with a PC that is not isolated.

To reduce the problems associated with non-isolation, the impedance between the inputs of the module and the ground point is made as high as possible. With a 3MΩ input impedance between the two input terminals, it was decided to use a 10MΩ resistance to ground at each of the amplifier inputs. This necessitates the use of high impedance JFET input amplifiers to buffer the AD625 whose input bias currents are too large to support this magnitude of resistance at the inputs (see below).

The only other aspect of the design to mention is provision of an attenuation and level shifting circuit to reduce the ±15V output of the AD625 to 0-2.5V needed by the AVR A/D converter. This is quite straightforward. It requires two stages of attenuation, the first to reduce the level so that level shifting will not push the signal beyond the limits of the operational amplifier. The level shifter uses a bridge arrangement to perform the first attenuation and subtraction of the offset voltage. The use of a 5.6V zener diode allows the provision of a temperature independent and clean reference voltage for the zero point.

The manual switches are sensed on a third switch bank to allow the microcontroller to read their settings. This is passed through a priority encoder (74LS148) to reduce the number of signals from four to two. The AC/DC switch is also sensed separately, giving three signals to be passed back. A multiplexer (74LS157) is used to select a second analogue channel on the same card.

Bias Current Considerations

Bias current flows between the amplifier inputs and ground. It is necessary to provide a path for this current to flow. In a differential amplifier there are bias currents flowing from both positive and negative inputs, and if equal resistances are provided from the inputs to ground, they cancel out – almost. The remaining difference is called offset current and is generally significantly smaller than individual bias currents.

When a differential amplifier is placed in a fixed circuit, the voltages at the output caused by the offset current can be nulled out by a trimming potentiometer or by careful design. In our case, where the circuit attached to the amplifier inputs can vary significantly from one measurement to another, it is not possible to provide a once-off nulling of the offset current. Though we provide equal resistances from the inputs to ground to cancel the bias currents, the actual resistance at those points varies depending on the external circuit in a DC measurement. The effect cannot be nulled out with our auto-zeroing circuit. The only possible way to estimate the effects is by manually turning off the voltages present in the external circuit. We still provide equal resistances to ground at the amplifier inputs as measurements in AC mode are effectively isolated from the external circuit by a capacitor, and so the measurements can be made accurately.

The solution is to use low drift, low bias current operational amplifiers to buffer the AD625 from the external circuit.

Grounding Considerations

The datasheets indicate that the ground connections on the two switches ADG409 and AD7512 are power grounds associated with the digital switching. The two amplifier devices have no power ground connections. Therefore it is sensible to link all the analogue grounds onto the same net, and to bring it to the analogue ground connection on the microcontroller. The connection to the common ground would occur through within the microcontroller. The analogue grounds occur at the 10M bias resistors, the reference input to the AD625, and in the following network. The two analogue signals should be shielded with the analogue ground and kept as remote as possible from the power nets.

Circuit

A gEDA adc-module.sch schematic file is also provided, along with the following symbols. These will need to be modified to provide the attributes necessary for producing a netlist and gerber file, and will be updated as time permits.