09/07/08

8

ES7

True RMS-to-DC Converters

Application notes

1. AC-coupled operation

Refer to the standard circuit of ES7 shown in Figure 2~3. ES7 will work in an

AC-coupled operation when the SW1 is opened. In AC-coupled operation, an

AC-coupled capacitor (Ccp) and bias resistors Rin must be required. For a low

frequency input under 100Hz, the Ccp need a 1uF or even larger capacitor to prevent

input signal from decaying.

Due to the architecture of ES7, a bias current is needed to activate the input buffer. The

resistor Rin applied from Vin to GND supplies a bias current flow path in AC-coupled

operation. The bias current flows from GND to Vin through Rin will cause a bias

voltage at Vin pin. So the Rin resistance should not be too large (cause an additional

zero offset) or too small (low input impedance).

2. Power Down Function

The ES7 provides a power-down enable pin (Pin 3). To enable the device, this pin must

be connected to –Vs or keep floating. If it is connected to V+, the device will enter

power-down mode.

should be connected as shown in figure 2. With post filter, the value of Cav should be

just large enough to give the maximum dc error at the lowest frequency of interest. And

the output ripple will be removed by the post filter.