•  
• EDN Europe MAGAZINE
•  
• Free SUBSCRIPTION
• Subscription Renewal
• View Media Kit

• EDN
• EDN Australia
• EDN Asia
• EDN China
• EDN Japan

Simple analog circuit provides voltage clipping and dc shifting for flash ADC

Many flash ADCs, such as National Semiconductor’s (www.national.com) ADC1175, have a recommended operating input-voltage range of 0.6 to 2.6V (Reference 1). However, in some applications, you must convert a symmetrical analoginput signal. The circuit in this Design Idea converts a symmetrical inputvoltage range of -0.2 to +0.2V into the recommended 0.6 to 2.6V range (Figure 1).The circuit also prevents the output voltage from going below –0.3V, which would probably damage the ADC.

The circuit uses an Analog Devices (www.analog.com) AD8002 dual-current- feedback operational amplifier to obtain a high bandwidth (Reference 2). The first block, noninverting amplifier IC1A, has a voltage gain of five. This block also provides high input impedance and low output impedance, so that the second block, IC1B, operates properly. The second block does most of the work. Starting from a basic inverting amplifier comprising IC1B, R4, and R5, you obtain the clipping effect by adding R3 and D1. R3, D1, R4, and R5 determine the clipping level. In addition, adding the IDC current dc-shifts the output voltage. You can trim adjustable potentiometer resistor P1 to obtain the desired output voltage shift—that is, 1.6V.

If diode D₁’s current is negligible, the output voltage, V_O, is -(1+R₂/ R₁)x(R₅/(R₃+R₄))xV_I+V_CCxR₅/ (R₆+P₁+R₇)=1.6-5xV_I. Given that the diode voltage, V_DIODE, is 0.6VS, V_O= -(R₅/R₄)xV_DIODE+V_CCxR₅/(R₆+P₁+R₇)=1.6-1.65=0.05V.

The clipping takes place near 0V, protecting the ADC. Raising the clipping level makes the circuit less linear in the nonclipping range. In other words, a design trade-off exists between clipping level and linearity. Resistor R8 limits the current through the ADC’s input pin. Capacitor C₂ is optional; it limits the VADC/VI bandwidth. Capacitor C₁ helps to reduce the voltage noise that might come from the –V_CC power supply.