Circuit provides constant current for ISFETs/MEMFETs

ISFETs and MEMFETs (ion-sensitive and membrane FETs) are solid-state chemical sensors that provide an electrical signal associated with the change of a chemical concentration in a solution. These types of sensors need a stable operating point. A common operating point is a drain-source current of 100 µA and a drain-source voltage of 500 mV. Under these conditions, the sensed chemical information is contained in the gate-source voltage, V_GS. Figure 1 shows a constant-current driver for ISFETs/MEMFETs. The current source, I, produces a voltage drop in resistor R₁; the voltage follower reflects this voltage to the drain-source terminals of the chemical sensor. In this example, V_DS is equal to IR. The entire current, I, traverses resistor R₁, because the op amp is an electrometer type with femtoamp-range input-bias current. The sensor's drain-source current, I_GS, is the difference between the constant currents, 2I and I – in this case, I. A REF200 provides the constant currents in the circuit in Figure 1. The REF200 has two internal 100-µA current sources and a current mirror.

Figure 1.	A constant-current driver provides the basis for a chemical-concentration test system.

Figure 2.	The gate-source voltage shows good correlation with the concentration of [H1] ions.

One of the 100-µA current sources provides I in Figure 1. The lower 2I current source uses the REF200's second 100-µA current source and the internal current mirror. The second voltage follower measures the gate-source voltage of the sensor – hence, the chemical concentration of interest. Figure 2 shows experimental results. You can see that the setup provides a linear relationship between gate-source voltage and the concentration of [H⁺] ions. The experiment used SiO₂ – and Si₃N₄ -based ISFET sensors. Figure 3 shows the variation of the threshold voltage as a function if pH.

Figure 3.	The gate-source voltage of an ISFET shifts with the degree of acidity.

Materials on the topic

1. Datasheet Texas Instruments OPA128
2. Datasheet Texas Instruments REF200