When making electrical impedance measurements, including bioimpedance measurements, electric impedance tomography (EIT), and focused impedance measurement (FIM), one or more electric currents are injected into the measurand. Depending on the application, one can choose to use a controlled voltage or a controlled current. For the cases where controlled current are used, it is important to have a well known current to be able to measure impedance or transfer-impedance accurately. To achieve this, one may use several different current sources that are already described such as [1, 2, 3, 4, 5, 6, 7, 8, 9].
An isolated current source is ideal to make independent current sources as required by FIM [10] since the outputs are independent of the inputs. In cases where measurements are done
The isolated analog current sources available today are either transformer-coupled [3, 4] or optically coupled. Optical isolation is beneficiary since it allows operation to DC and is insensitive to electromagnetic fields. There are two main types of optically isolated current sources: The first is the use of one or more optocouplers to transfer a signal from the input to an opamp-based current source on the output side [8]. The second type is an H-bridge as presented in [9]. The current source presented here is of the second type. The novelty of the presented circuit is that it uses the optocouplers output currents directly. This is possible since linear optocouplers are used. Another important difference from other H-bridge circuits is that the presented circuit eliminates drift and nonlinearities due to aging or temperature.
This circuit is based on a particular type of optocoupler that is shown in figure 1. Necessary description to understand the presented circuit is as follows: The light emitting diode (LED) produces light that falls on two closely matched photodiodes,
The component, voltage, and current designators in this section refers to figure 2. To simplify, we define
The circuit works as follows: The input signal is
These signals are then fed to the inputs of the four opamps driving the optocouplers as shown in figure 2. The optocoupler circuits convert the input voltages to currents in the output photodiodes. The gain in this step is given by
The currents flowing out of the optocouplers are now given by
where
The optocouplers are connected so that when there is no input, i.e
The power supplies for the input are balanced regulated supplies (both positive and negative supply) of equal magnitude. The power supplies for the output should have the same voltage difference from positive to negative supplies as the power supplies on the input to ensure as similar conditions for
The maximum photodiode current is approximately 87 μ
Figure 2 does not show all details of the circuit creating the differential signal or the bias circuit as this is regarded as outside the scope of this article. The details of the circuits driving the optocoupler is also left out as this information can be found in [12, 14, 15].
If we assume that the amplifier made of
Two variants of the prototype was made, with different opamps. The opamps had different Gain-Bandwidth (
The datasheet for the optocoupler or the SPICE model do not give necessary data to calculate the output impedance accurately, but the data available suggest output impedances in the range 10
The output noise of this circuit is dependent on noise contributions from all devices and on the frequency dependent transfer function to the output. We have not made a generic noise analysis that is including all noise sources due to the high complexity in such calculations. Instead we have made a simplified noise analysis. The simplified noise analysis is shown in equation 10. This is a noise analysis where
The noises are calculated using equation 11 where
Equations 10 and 11 show us that if the resistance in
The measured output impedance is shown in figure 3.
Simulations of the circuit with different opamps gave results shown in table 1.
Measured and simulated bandwidths, unit for all parameters is
Opamp | Opamp GBW | Sim. or Meas. | BW |
---|---|---|---|
TLC274 | 1.7 | Measured | 0.68 |
TLC274 | 1.7 | Simulated | 1.28 |
TS924 | 4 | Measured | 1.35 |
TS 924 | 4 | Simulated | 2.42 |
LT1214 | 28 | Simulated | 6.90 |
TLC274 is from Texas Instruments, TS924 is from ST Microelectronics, and LT1214 is from Linear Technology.
The data sheet for the Avago Technology optocoupler states that the LED bandwidth is 9
We have demonstrated that it is feasible to operate the circuit up to 1
Optimization of bandwidth means choosing opamp U2 that is fast and powerful enough to drive the LED, and that does not contribute too much to the parasitic capacitance to the node at the anode of
Bandwidth as high as the presented circuit is capable of can be useful for electrical bioimpedance measurements, including bioimpedance spectroscopy.
The presented circuit has the potential to outperform the noise performance of other circuits based on linear optocouplers since it uses an absolute minimum of noise-contributing components on the output side. By careful design, the main noise contributor will be
There are a number of alternatives that have not been explored since this article is only aiming to present the basic idea for the new circuit. Examples of alternatives are: Use of only one output branch, automatic adjustment of cancel mismatch in the optocouplers, several sources driven by
The optocouplers are used in photoconductive mode, which requires supplies on the output side. One could also use the optocouplers in photovoltaic mode to eliminate the need for a supply on the output side, but the available energy is very limited, so the bandwidth would be low.
The presented circuit is a galvanically isolated voltage controlled current source. It is made using linear optocouplers with an absolute minimum of components in the isolated output section. Thanks to no components on the output side, the circuit has potential for low noise. The isolation makes it suitable for use in medical purposes such as bioimpedance, FIM, EIT, or nerve stimulation.