Circuit functions and advantages

Digitally controlled current sources are critical in many applications such as power management, solenoid valve control, motor control, impedance measurement, sensor excitation, and pulse oximeters. This article introduces three current sources that use DACs, op amps, and MOSFET transistors to support digital control of a serial interface.

The DAC chosen is a high-resolution (14 or 16-bit), low-power CMOS with a standard serial interface. The 16-bit DAC AD5543 is available in ultra-compact (3 mm × 4.7 mm) 8-lead MSOP and 8-lead SOIC packages. The 14-bit DAC AD5446 is available in a small 10-pin MSOP package. Both DACs are compatible with most DSP interface standards and are SPI, QSPI and MICROWIRE compatible. The external reference voltage input allows many variations in output level, up to 10 V.

The device combination achieves industry-leading small PC board area, low cost, and high resolution. All three designs provide low-risk solutions and use industry-standard components.

Circuit description

All three circuits require a single 5 V supply for the DACs and a ±15 V supply for the op amps. Some circuits may require an accurate external voltage reference (see tutorial MT-087 ).

Each circuit has two levels. The first stage is the input stage, which is composed of DAC and operational amplifier. The second stage is an N-channel MOSFET transistor output stage (Figures 1 and 2), which provides current in response to the digital word sent to the system.

As shown in Figure 1, the input stage of the circuit consists of a current output DAC (AD5446) and an operational amplifier ( AD8510 ). It converts the command word and drives the transistor, in addition to modulating the voltage applied to the single resistor. Command words are sent through the SPI interface.

The output stage consists of an N-channel MOSFET transistor ( NTE4153N ), which can provide higher current than the op amp and single resistor output. Single resistor R1 produces current when voltage is applied to its pin. A transistor regulates this current.

The load current is:

where D is the decimal representation of the digital word loaded into the DAC. However, R _DAC >> R1 (R _DAC is nominally 9 kΩ), so the load current can be approximately expressed as:

When R1 = 100 Ω and V _IN = 5 V, I _LOAD is programmable from 0 mA to 50 mA with a resolution of 3 μA (1 LSB at 14 bits). The output supply voltage is approximately 20 V, limited by the MOSFET transistor breakdown voltage. The ADR425 is a low power 5 V precision reference that is ideal for this circuit.

The circuit shown in Figure 2 also uses the AD5446 DAC. However, at this point the DAC operates in "inverse" or voltage mode, providing a voltage output using a 1.2 V reference such as the ADR512 .

The DAC output voltage is available on pin 9 and ranges from 0 V to 1.2 V. For more information on the "reverse" voltage operating mode, please refer to the AD5446 data sheet.

The operational amplifier used in this circuit is the OP1177 , which is a high-precision, very low offset voltage (maximum 60 μV) device. When the DAC operates in voltage output mode, low offset voltage is very important.

N-channel MOSFET transistors and operational amplifiers together form a high-current output follower circuit.

Negative feedback from the transistor source pin to the op amp input regulates the amount of current flowing through R1. The load current is:

When R1 = 10 Ω and V _IN = 1.2 V, I _LOAD is programmable from 0 mA to 120 mA with a resolution of 7 μA (1 LSB at 14 bits).

The third circuit, shown in Figure 3, uses a 16-bit DAC AD5543 as the input stage and a Howland current pump circuit as the output stage. Howland current pumps have two advantages over MOSFET outputs: high output impedance and the ability to provide bipolar output current. To improve stability, the circuit is generally symmetrical. Therefore, R1 = R1', R2 = R2', R3 = R3'.

I _LOAD is programmable from 0 mA to 20 mA with 300 μA resolution (1 LSB at 16 bits) when R1 = 150 kΩ, R2 = 15 kΩ, R3 = 50 Ω, and V _IN = 5 V , and the circuit has a very high output impedance.

To properly separate the DAC from the op amp and achieve ideal performance, excellent layout, grounding, and decoupling techniques are necessary for all three circuits. (Please refer to tutorials MT-031 and MT-101 ).