Texas Instruments THS4601 Evaluation Module THS4601EVM THS4601EVM Datenbogen

3-4

prefers, an external potential can be introduced at J7, and use only resistors
R7 and R5. If the source of VS+ in the system is noisy, and there is a less noisy
potential available in the system, it can be connected to J7. R7 and R5 can then
be used as a voltage divider off the reference attached to J7 to create the VS+
divided-by-2 potential.

The gain measured from J1 to J3 is 1 in the passband, and is still determined
by:

Vout

V

in

*

+ *

Z5

R3

(in the passband of the stage).

The circuit provides approximately 50 

Ω

 of input impedance within the

passband, as determined by R1 || R3 (because R3 is at a virtual ground
potential due to the action of the ideal op amp model and the decoupling
capacitor Z2).

At first sight this may not appear to be an amplifier at all, but in reality the input
is a current and the gain of the amplifier is measured in 

Ω

, rather than volts/volt.

This configuration is often used in applications where the output of a
transducer is current or charge, i.e., photodiode. Applications include optical
power measurements in optical networks, x-ray machines (measuring light
power once the x-rays have been converted to light), and photomultiplier tube
amplifiers.

Due to the capacitance associated with the transducer, a pole has to be
created in the feedback loop to ensure stability.

The wide bandwidth and high operating voltage capability of the THS4601
make it unique in the applications by enabling it to offer wide output swings,
thereby delivering higher dynamic ranges in a single stage.

Typically, design of a transimpedance circuit is driven by the characteristics of
the current source that provides the input to the gain block. A photodiode is the
most common example of a capacitive current source that would interface with
a transimpedance gain block. Continuing with the photodiode example, the
system designer traditionally chooses a photodiode based on two opposing
criteria: speed and sensitivity. Faster photodiodes cause a need for faster gain
stages, and more sensitive photodiodes require higher gains, in order to
develop appreciable signal levels at the output of the gain stage.

These parameters affect the design of the transimpedance circuit in a few
ways. First, the speed of the photodiode signal determines the required
bandwidth of the gain circuit. However, the required gain, based on the
sensitivity of the photodiode, limits the bandwidth of the circuit. Additionally,
the larger capacitance associated with a more sensitive signal source  detracts
from the achievable speed of the gain block. The dynamic range of the input