ADIAD8436 High Precision RMS to DC Conversion Solution

ADI's AD8436 is a new generation of translinear high-precision low-power true RMS/DC converter, which can accurately calculate the rms value of AC waveforms, including complex terms produced by switching power supplies and TRIAC. The accuracy is better than 0.5%, the output offset is less than 10uV, the error when the crest factor (CF=1-10) is less than 0.5%, the input range is 100 μV rms to 3 V rms (8.5 V pp), the dual power supply voltage is ±2.4 V to ±18 V, and the single power supply voltage is 4.8 V to 36 V. It is mainly used in remote control, handheld instruments, power supplies and automobiles with limited board space and outdoor environments. This article introduces the main features of AD8436, functional block diagram, RMS core block diagram, crest factor test circuit diagram, typical application circuit, AD8436-EVALZ evaluation board circuit diagram and PCB layout diagram.

The AD8436 is a new generation, translinear precision, low power, true rms-to-dc converter that is loaded with options. It computes a precise dc equivalent of the rms value of ac waveforms, including complex patterns such as those generated by switchmode power supplies and triacs. Its accuracy spans a wide range of input levels and temperatures. The ensured accuracy of ≤±0.5% and ≤10 μV output offset result from the latest Analog Devices, Inc., technology. crest factor error is <0.5% for CF values ​​between 1 and 10.

The AD8436 delivers instant true rms results at less cost than misleading peak, averaging, or digital solutions. There is no programming expense or processor overhead to consider, and the 4 mm × 4 mm package easily fits into those tight applications.

On-board buffer amplifiers enable the widest range of options for any rms-to-dc converter available, regardless of cost. For minimal applications, only a single external averaging capacitor is required. The built-in high impedance FET buffer provides an interface for external attenuators, frequency compensation, or driving low impedance loads. A matched pair of internal resistors enables an easily configurable gain-of-two or more, extending the usable input range even lower. The low power, precision input buffer makes the AD8436 attractive for use in portable multi-meters and other battery-powered applications. The precision dc output buffer offers extremely low offset voltages, thanks to current bias cancellation.

Unlike digital solutions, the AD8436 has no switching circuitry limiting performance at high or low amplitudes (see Figure 2). A usable response of <100 μV and >3 V extends the dynamic range with no external scaling, accommodating the most demanding low signal conditions. The AD8436

operates from single or dual supplies of ±2.4 V (4.8 V) to ±18 V (36 V). A and J grades are available in a compact 4 mm × 4 mm, 20-lead chip-scale package. The operating temperature ranges are −40°C to 125°C and 0°C to 70°C.

The AD8436 is a good choice for remote, portable battery-powered instrumentation, energy and automotive applications where limited board space and outdoor environments are considerations.

AD8436 key features:

Computes true rms value instantly

Accuracy: ±10 μV ± 0.5% of reading

Wide dynamic input range

100 μV rms to 3 V rms (8.5 V pp) full-scale input range

Larger inputs with external scaling

Wide bandwidth:

1 MHz for −3 dB (300 mV)

65 kHz for additional 1% error

Zero converter dc output offset

No residual switching products

Specified at 300 mV rms input

Accurate conversion with crest factors up to 10

Low power: 300 μA typical at ±2.4 V

Fast settling at all input levels

High-Z FET separately powered input buffer

RIN ≥ 1012 Ω, CIN ≤ 2 pF

Precision dc output buffer

Wide supply range

Dual: ±2.4 V to ±18 V

Single: 4.8 V to 36 V

Small size: 4 mm × 4 mm package

ESD protected

图1。AD8436功能方框图

图2。AD8436 RMS核方框图

图3。波峰因素测试电路图

图4。AD8436典型应用电路

AD8436-EVALZ评估板

The AD8436-EVALZ provides a platform to evaluate AD8436 performance. The board is fully assembled, tested and ready to use after the power and signal sources are connected. Figure 45 is a photograph of the board. Signal connections are located on the primary and secondary sides, with power and ground on the inner layers.

The AD8436-EVALZ offers many options, without sacrificing simplicity. The board is tested and shipped with a 10 μF averaging capacitor (CAVG), 3.3 μF low-pass filter capacitor (C8) and a 0.1 μF (COPT) capacitor to optimize crest factor performance. To evaluate minimum cost applications, remove C8 and COPT.

图5。AD8436-EVALZ评估板外形图

图6。AD8436-EVALZ评估板电路图

图7。AD8436-EVALZ评估板元件布局图
详情请见：
http://www.analog.com/static/imported-files/data_sheets/AD8436.pdf