Design Ideas

February 3, 1997

Cascode circuit lessens DAC degradation

Mark Shill, Burr-Brown, Tucson, AZ

Current-output D/A converters usually deliver their best accuracy if the current-output (summing-junction) pin remains at a constant dc voltage, because of the finite output impedance of the current-out DAC output (Figure 1). Two sources contribute to the output impedance. One is the impedance of the R-2R ladder used to binary-scale the bit currents; the second is the nonlinear output impedance of the transistor current switches. The R-2R ladder impedance, typically several kilohms, can cause significant offset and gain errors if the I_OUT pin drives resistive loads heavier than 100ohms. Next, the current switches' output impedance, usually tens of megohms, can cause degraded integral linearity at the 14- to 16-bit level because of nonlinearity with voltage. Figure 2 shows a method that allows a current-output DAC to drive a load while keeping the summing junction at a constant voltage.

Typically, you use most current-output DACs with an external I/V-converter op amp (Figure 3). The load is situated in the feedback path of the op amp, such that the DAC's output current flows through it. The I/V converter keeps the DAC's summing junction at a virtual ground, thus preventing any degradation of the DAC's transfer function. However, in some applications, it's desirable to transmit the DAC's current over a long twisted-pair line to a remote load, such as an actuator or a bridge. In this case, the I/V converter becomes ineffective because of the series impedance of the wire to the remote load. If the wire's impedance approaches 100ohms or greater, the DAC's summing junction no longer stays at virtual ground, because of the current-dependent voltage drop in the wire.

In Figure 2, the DAC700 is a unipolar, current-output DAC with an output range of 0 to –2 mA. It has an internal R-2R ladder impedance of approximately 4 kilohms, which would cause an additional 0.25 mA of output-current gain error for every 1V of compliance-voltage swing at the I_OUT terminal. The circuit comprising op amp IC₁ and n-channel MOSFET Q₁ maintains the I_OUT terminal at a fixed 0V. The op-amp-driven cascode, Q₁, keeps the summing junction at 0V and allows the DAC's output current to flow through the remote-load impedance.

The remote load receives its power from a REF102 10V reference. Q₁'s drain voltage can vary from 10 to approximately 0V, depending on the programmed code to the DAC. For all conditions, IC₁ adjusts the gate-drive voltage to Q₁ to maintain the source terminal at 0V, even if Q₁ goes into the triode region as its drain voltage approaches 0V. Note that for proper operation, the following relationship must hold: I_DAC(R_WIRE+R_LOAD)<10V, where I_DAC is the maximum DAC output current.

If the voltage swing across the driven impedance exceeds 10V, Q₁ enters the cutoff region and can no longer conduct. For cases involving large compliance-voltage swings, you can use a larger remote-supply voltage. If so, you must take care in choosing MOSFET Q₁ to ensure that its drain-source breakdown voltage is greater than the maximum drain-terminal voltage. The circuit in Figure 2 uses a Siliconix SD215 n-channel, enhancement-mode MOSFET, whose drain-source breakdown voltage is greater than 20V.

You can use this method to drive any load that produces large compliance-voltage swings at the current-output pin. For best results, use an op amp with high bandwidth, such as the OPA627. If you use a current-output DAC with high output capacitance, such as a CMOS device, you may need capacitor C_OPT to ensure stability. (DI #1982)

FIGURE 1

The R-2R ladder network and the switches' nonlinear output impedances make up the output impedance of a current-output DAC.

FIGURE 2

An op amp and an n-channel MOSFET collaborate to keep the DAC's summing-junction pin at virtual ground, thereby improving the transfer function's linearity.

FIGURE 3

In remote-load configurations, voltage drops in the connecting wire render the I/V converter's virtual ground nonvirtuous.