How to Choose a Capacitance-to-Digital Converter: AD774x Selection Guide

Bottom Line: When selecting a capacitance-to-digital converter (CDC) for precision sensing applications, the three most critical factors are measurement resolution (target ≥ 20-bit effective for sub-femtofarad detection), the number of capacitance input channels (single-ended vs. differential), and automotive or industrial qualification. Analog Devices' AD774x family — the AD7745ARUZ-REEL7, AD7745ARUZ, AD7746ARUZ, and AD7747ARUZ — delivers 24-bit Σ-Δ CDC performance in a compact 16-pin TSSOP, making it the benchmark reference for humidity sensors, level gauges, and proximity detectors worldwide.

What Is a Capacitance-to-Digital Converter?

A capacitance-to-digital converter (CDC) measures the capacitance of a sensor element and outputs a digital word proportional to that capacitance. Unlike a simple RC oscillator approach, a Σ-Δ CDC continuously charges and discharges the sensor capacitance into a known reference, accumulating charge differences that cancel offsets and drift. The result is 24-bit resolution with noise levels below 4 aF (attofarad) RMS. CDCs are preferred over voltage-mode bridges in applications requiring sub-picofarad accuracy over a −40 °C to +125 °C temperature range.

Key Selection Parameters

1. Resolution and Noise Floor

Resolution determines the smallest capacitance change the converter can reliably detect. The AD774x family achieves 24-bit resolution with an RMS noise of approximately 4 aF at the slowest output data rate (OVR = 256, approximately 9.1 samples/s). At higher data rates (OVR = 8, approximately 90.9 samples/s), noise rises to about 20 aF RMS — still well below the picofarad-level changes typical of proximity or humidity sensors. For applications demanding sub-femtofarad discrimination, select OVR ≥ 64 and calibrate the offset using the on-chip DAC capacitors. Competing 16-bit CDC solutions from alternate vendors typically exhibit noise floors of 200 aF or higher, a 50× penalty.

2. Number of Input Channels

The number of independent capacitance inputs determines how many sensor elements you can multiplex without external switching. The AD7745ARUZ and AD7745ARUZ-REEL7 provide one differential CDC channel (CIN1+/CIN1−), making them ideal for single-element differential sensors such as a push-pull capacitive proximity switch. The AD7746ARUZ adds a second channel (CIN2+/CIN2−), enabling simultaneous measurement of two sensors or a sensor-plus-reference configuration that compensates for parasitic capacitance variations. The AD7747ARUZ offers a single-ended capacitance input (CIN+) plus a temperature sensor, suited for humidity element characterization at design-time. Choosing between one and two channels avoids the cost and timing jitter of an external multiplexer.

3. Temperature Measurement Capability

Integrated temperature measurement allows a single chip to compensate capacitance readings for thermal drift without an external thermistor or RTD. The AD7745 and AD7746 include an on-chip temperature sensor accurate to ±2 °C over the −40 °C to +85 °C range. The AD7747 routes its temperature measurement pin externally to allow a remote thermistor, enabling ±0.5 °C accuracy when paired with a 10 kΩ NTC element. In industrial flow-meter designs where fluid temperature shifts by 80 °C, this integrated compensation can eliminate drift errors of up to 50 fF — equivalent to 5 LSBs at 24-bit full-scale.

4. Input Capacitance Range and EXC Amplitude

The EXC pin generates a square-wave excitation signal applied to the CDC sensor. The AD774x family supports ±1.25 V, ±1.0 V, or ±0.5 V excitation amplitude (register-programmable), and the typical input capacitance range is 0 to 17 pF with up to 17 pF of parasitic capacitance on the shield (SHLD) pin. Applications measuring large-area touch panels (>50 pF full-scale) require adding an external capacitance subtraction network or switching to a higher-range CDC. For lab-scale humidity or pressure sensors with full-scale spans of 2–15 pF, the AD774x range is a direct fit. Verify that your PCB trace parasitics remain below the 17 pF limit by keeping sensor traces under 40 mm and routing them on the component side with a guard ring tied to SHLD.

5. Digital Interface: I²C vs. SPI

All AD774x devices communicate via I²C (up to 400 kHz fast mode), making them easy to chain on a 2-wire bus shared with humidity or pressure sensors. The 7-bit I²C address is fixed at 0x48, so a maximum of one device per bus segment — designs requiring multiple CDC channels on a single MCU must use the AD7746's two-channel option or add a hardware I²C multiplexer (e.g., TCA9548A). There is no SPI-native CDC in this family; if your system is SPI-only, evaluate the AD7152 (2-channel, SPI/I²C, 12-bit) or add a software I²C bit-bang. Budget approximately 4 ms per conversion cycle at the default OVR setting when calculating polling loop timing.

6. Power Consumption and Supply Voltage

Power budget directly affects battery-operated IoT nodes and wearable devices. The AD774x operates from 2.7 V to 5.25 V and draws typically 700 µA active current, dropping to 1.5 µA in power-down mode. At a 1-second measurement interval with 4 ms active and 996 ms in power-down, the average current falls to approximately 4 µA — enabling a CR2032 coin cell (225 mAh) to sustain over 5 years of operation. In always-on industrial applications running continuous conversions, the 700 µA draw over a full year amounts to about 6.1 Wh, negligible for a 24 V panel-powered transmitter but significant for a wireless battery-backed sensor node.

7. Packaging and Tape-and-Reel Options

Tape-and-reel packaging simplifies high-volume SMT assembly by enabling automated pick-and-place without manual component handling. The AD7745ARUZ-REEL7 is identical to the AD7745ARUZ in silicon but supplied on 7-inch (approximately 1,000-unit) reels — the "REEL7" suffix denotes the 7-inch reel format versus the standard 13-inch reel. For quantities under 500 units, the tube/tray form of AD7745ARUZ is usually more cost-effective. Above 1,000 units per production run, the REEL7 variant reduces kitting labor and minimizes ESD exposure during handling. Confirm with your contract manufacturer whether their pick-and-place feeder is compatible with the 7-inch reel pitch before placing bulk orders.

Recommended Products Comparison Table

Product	Channels	Temperature Sensor	Supply Voltage	Packaging	Best For
AD7745ARUZ-REEL7	1 differential	On-chip ±2 °C	2.7–5.25 V	16-TSSOP, 7" T/R	High-volume SMT, single-sensor
AD7745ARUZ	1 differential	On-chip ±2 °C	2.7–5.25 V	16-TSSOP, tube	Prototyping, low-volume
AD7746ARUZ	2 differential	On-chip ±2 °C	2.7–5.25 V	16-TSSOP, tube	Dual-sensor or sensor+reference
AD7747ARUZ	1 single-ended	External thermistor	2.7–5.25 V	16-TSSOP, tube	High-accuracy temp compensation

Selection Decision Flowchart

Step 1 — How many sensor elements?

• 1 sensor → go to Step 2
• 2 sensors → choose AD7746ARUZ (2-channel differential CDC)

Step 2 — Do you need external thermistor compensation (±0.5 °C)?

• Yes → choose AD7747ARUZ
• No → go to Step 3

Step 3 — Is this a high-volume (>1,000 units/run) SMT production?

• Yes → choose AD7745ARUZ-REEL7 (7-inch tape-and-reel)
• No → choose AD7745ARUZ (tube packaging, lower MOQ cost)

Frequently Asked Questions

What is the maximum capacitance range of the AD774x CDC? The AD774x family measures capacitances from 0 to ±4 pF (differential) or 0 to 8 pF (single-ended) relative to an internal baseline, with up to 17 pF of parasitic capacitance tolerated on the EXC/SHLD pins. Full-scale output corresponds to the programmed nominal capacitance plus the ±4 pF swing range. Sensors with full-scale spans larger than 17 pF total require an external offset network. Always consult the AD7745/AD7746 datasheet (Rev. E) for precise register-level capacitance mapping.

Can I use the AD7745 with a 3.3 V microcontroller? Yes. All AD774x devices support a 2.7 V to 5.25 V supply, and the I²C bus is compatible with 3.3 V logic when the host microcontroller's I²C pull-up voltage matches. Use 4.7 kΩ pull-up resistors to 3.3 V on SDA and SCL. The EXC amplitude should be set to ±1.0 V (register bits EXCA[1:0] = 01b) to keep capacitive drive within limits at 3.3 V supply. No level shifter is required in a pure 3.3 V system.

What is the difference between AD7745ARUZ and AD7745ARUZ-REEL7? These two part numbers are electrically and functionally identical — same silicon, same 24-bit CDC, same 16-pin TSSOP footprint. The only distinction is packaging format: AD7745ARUZ ships in tube or tray form (typically 96 pieces per tube), while AD7745ARUZ-REEL7 ships on a 7-inch tape-and-reel (typically 1,000 pieces per reel) for automated SMT placement. Choose REEL7 for volume assembly lines and the plain ARUZ for bench evaluation or small-batch production.

How does the AD7746 handle simultaneous dual-channel measurement? The AD7746 contains two independent capacitance front-ends sharing a single Σ-Δ modulator. In continuous conversion mode, the device interleaves measurements between CIN1 and CIN2 at half the single-channel output data rate. At OVR = 256 (single-channel rate 9.1 sps), dual-channel operation delivers approximately 4.5 sps per channel — sufficient for most slow-varying humidity or level sensing applications. If you need simultaneous high-speed sampling on both channels, consider two AD7745 devices with synchronized CNVST signals.

Is the AD774x AEC-Q100 qualified for automotive use? The standard AD7745ARUZ and AD7746ARUZ are qualified for industrial temperature range (−40 °C to +85 °C) but are not marked AEC-Q100 Grade 1. For automotive applications requiring Grade 1 (−40 °C to +125 °C) or Grade 0 (−40 °C to +150 °C) qualification, contact Analog Devices' automotive sales channel or search FindMyChip's /search catalog for AEC-Q100 screened alternatives. Always verify the date code and lot traceability when sourcing automotive-grade CDC components through distribution.

Conclusion and Next Steps

Selecting the right capacitance-to-digital converter starts with two questions: how many sensor elements you need to measure simultaneously, and whether your production volume justifies tape-and-reel packaging. For single-element applications, the AD7745ARUZ or its reel variant AD7745ARUZ-REEL7 provides the best balance of 24-bit resolution, low power, and straightforward I²C integration. Two-element designs benefit from the AD7746ARUZ, while remote-thermistor temperature compensation points to the AD7747ARUZ.

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