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DC Drift in Enhanced Howland Current Source build with ADA4857 and ADA4817

Question asked by liubenyuan on Apr 16, 2017
Latest reply on Apr 17, 2017 by liubenyuan

I build a Howland current source that can pump bi-directional current up to 1MHz, with maximum output swing to +-10V. The schematic simulated using LTSpice is shown below.

 

This circuit is also being used in analog dialogue 2009, entitled as,

Difference Amplifier Forms Heart of Precision Current Source | Analog Devices 

where ADA4857 and R1, R2, R3, R4 together can be replaced by a difference amplifer such as AD8276 or AD8274. However, these difference amplifier (AD8276, AD8274) has low bandwidth and small slew rate, which might not suit our needs.

 

The enhanced howland current source is:

With R1=R2=R3=R4, and the output current is approximately I = V / Rset. ADA4857 has excellent CMRR and is used to build this difference amplifer, AD4817 has low bias current and low input offset, which is used as an unit gain buffer.

 

However during simulation, there is a dc offset at the node sense and out.

 

  1. How could I remove this dc offset?
  2. and Is there any recommendations when layout the PCB of enhanced howland current source using the amplifiers from analog ?

 

Thanks!

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