HF2CA Current Amplifier Data Sheet

This chapter contains the data sheet of the HF2CA Current Amplifier which is a preamplifier dedicated to the HF2 Series instruments. This data sheet is distributed only as part of the HF2 User Manual, and therefore not available separately.

The content of the chapter starts with the list of key features of the preamplifier, and continues with sections including the specifications, the detailed functional description, several possible applications, and finally an extended recommendation for 3rd party cables and connectors.

Key Features

  • Current amplifier for high capacitive loads - shunt resistor based

  • Voltage amplifier with selectable gain 1 or 10

  • Input impedance switchable between 10 V/A and 1 MV/A

  • Bandwidth from DC up to 100 MHz

  • 2 differential amplification channels with switchable AC/DC coupling

  • Adjustable output gain of 1 or 10

  • Very low noise and small input leakage

  • Single connector for power supply and control

The HF2CA current amplifier converts a differential input current to a differential output voltage in a wide frequency range. This device functions as an active probe and is conveniently placed close to the measurement setup. It supports applications with high capacitive loads such as dielectric impedance spectroscopy. When no shunt resistor is selected, the current amplifier works as a voltage amplifier. The careful design of the HF2CA insures stable operation over the entire frequency range.

hf2ca function
Figure 1. HF2CA functional overview


Unless otherwise stated, all specifications apply after 30 minutes of device warming up.

Table 1. General
Parameter Description


100 x 60 x 25 mm


0.4 kg

storage temperature

-20 °C to 65 °C

operating temperature

5 °C to 40 °C

specification temperature

25 °C

specification supply voltage

12 V


4 SMB inputs, 2 SMB outputs, 1 RJ45 (no Ethernet)

Table 2. Specifications
Parameter min typ max

positive supply voltage VDD+

12 V

15 V

20 V

negative supply voltage VDD-

-20 V

-15 V

-12 V

supply current

60 mA

80 mA

120 mA

frequency response

frequency range



100 MHz

frequency range (AC coupled)

100 Hz


100 MHz

small signal bandwidth / 3dB cut-off (0.1 VPP input, 50 pF output load, gain 1)

100 MHz



small signal bandwidth / 3dB cut-off (0.1 VPP input, 50 pF output load, gain 10)

25 MHz



large signal bandwidth / 3dB cut-off (1 VPP, 50 pF)

40 MHz




input voltage noise (10 kHz)


7 nV/√Hz


input voltage noise (10 MHz)


6 nV/√Hz


input bias current


2 pA

10 pA

transimpedance gain (equivalent to input impedance)

10 V/A


1 MV/A

transimpedance gain accuracy (G=1)


±0.1 %


transimpedance gain accuracy (G=10)


±1 %


input offset voltage



1 mV

common-mode offset range

-10 V


7.5 V


output voltage gain




control interface

input high level

2.0 V


5 V

input low level

0 V


0.8 V

all transitions on SDI, SDO, SCK, SLC



1 μs

SCK clock period

10 μs



SDI data to clock setup tDS

2 μs



SDI data hold from clock tDH

1 μs



SLC clock to latch setup tLS

1 μs



SLC latch hold tLH

10 μs


20 μs

SCK clock free time tCF

20 μs



zctrl digital interface
Figure 2. Digital control timing

Some parameters depend on the transimpedance gain settings. The following table provides an overview. The values in this table are typical values, they depend on the source capacitance, on the input signal swing, and as well as on the capacitive load on the output of the amplifier.

Table 3. Gain dependent parameters
Input impedance setting Bandwidth / 3dB cut-off frequency Maximum input current range Maximum input current noise

10 V/A

100 MHz

±160 mA

400 pA/√Hz

100 V/A

50 MHz

±16 mA

42 pA/√Hz

1 kV/A

5 MHz

±1.6 mA

5.6 pA/√Hz

10 kV/A

500 kHz

±160 μA

1.3 pA/√Hz

100 kV/A

50 kHz

±16 μA

400 fA/√Hz

1 MV/A

5 kHz

±1.6 μA

128 fA/√Hz

hf2 ca dimensions
Figure 3. Casing dimensions of the HF2CA

Functional Description

The HF2CA external amplifier can be placed close to the signal source whereas the HF2 Instrument can be several meters away. Such a setup significantly improves the measurement quality due to less parasitics effects and to smaller interferences.

The two signal channels of the HF2CA can be used as separate amplification channels, or alternatively, in differential mode connected to the differential input of the HF2 Instrument.

hf2 ca detailed schematic
Figure 4. Detailed block diagram

Input and Output

Shunt resistors: HF2CA measures the current between the positive and the negative input terminal by measuring the voltage drop across a resistor, that is shunted between the inputs (see Figure 4) . The supported resistor values are given in Table 3 . It is also possible to remove all internal resistors and to support any custom resistor that is externally connected (see Custom Input Impedance). When all resistors are removed (infinite impedance), then the HF2CA becomes a voltage amplifier with selectable gains 1 and 10.

JFET input amplifiers: the HF2CA is based on JFET input amplifiers that provide very low-noise over a wide frequency range. Additionally, the ultra-low input bias current of typically 2 pA allows for precise current measurements at small signal amplitudes. The input voltage range of the JFET input amplifiers is -10 V to 7.5 V for each input which is also the common mode offset range.

Single vs. differential mode: a selectable switch to amplifier ground allows the user to earth the negative terminal of each input and to operate in single-ended mode without needing external circuits. Alternatively, when leaving the ground switches open, it is possible to use a differential input signal or to connect the negative terminals to local ground externally.

AC vs. DC mode: a selectable switch after the input amplifiers allows the user to measure DC or close to DC signals, or when this is not required, to select AC coupling with a cut-off frequency at 100 Hz and eliminate potential 50/60 Hz noise from the measured signal.

Power Supply and Remote Control

The HF2CA is designed for use with the HF2 Series with its differential signal for improved signal-to-noise, and a single cable that provides power and control signals. A straight-through (as opposed to cross-over) Ethernet cable must be used. The cable carries the following signals:

  • Power: positive and negative supply, ground

  • Digital control: SDI digital input signal to control the preamplifier settings, SDO output signal for device detection (details of function not disclosed to users), SCK clock signal, and SLC latch signal. SDI, SCK and SLC are used to program the shift registers on the amplifier and thereby adjust the correct settings. The setting bits are given in Table 4. The timing diagram of the digital interface is given in Figure 2. The MSB of the register settings is shifted in first.

Table 4. HF2CA register settings
Register bit Name Description

15 to 10





0: set output gain to 1

1: set output gain to 10



0: set AC coupling for input 2

1: set DC coupling for input 2



0: set AC coupling for input 1

1: set DC coupling for input 1



0: set differential operation

1: set single-ended operation



1: set resistor 1 MV/A



1: set resistor 100 kV/A



1: set resistor 10 kV/A



1: set resistor 1 kV/A



1: set resistor 100 V/A



1: set resistor 10 V/A


  • Impedance spectroscopy

  • Large capacitive loads

  • Wheatstone-bridge configuration

  • Preamplifier for HF2IS impedance spectroscope and HF2LI lock-in amplifier

Differential Current Measurement with Common-mode Offset

The resistors at the input of the amplifier can be inserted in a current path as shown in the figures. With this, fast current transients can measured at large common-mode voltages, which are in the range from -10 V to 7.5 V are possible. This is used in, e.g., high-energy physics to record the radiation-induced current in a photo diode.

hf2 application user offset
Figure 5. HF2CA differential current measurement

Multi-frequency Impedance Spectroscopy

HF2 application microfluidic
Figure 6. HF2CA impedance spectroscopy

The HF2CA in combination with the HF2IS impedance spectrometer is the solution to measure impedance in, for example, flow-through microsystems. The challenge here is to measure the channel impedance at high frequencies (>10 MHz). The large capacitance occurring at electrode electrolyte interfaces can lead to stability issues in a transimpedance amplifier. A solution is to use the electrodes in a Wheatstone bridge configuration with shunt resistors. The HF2CA offers this solution.

As shown in the figure, electrodes are place on the channel walls of a microfluidic channel (width in the order of 20 to 50 μm). Electrodes E1 and E2 are stimulated with a sinusoidal voltage, the electrodes E3 and E4 are connected to the positive amplifier inputs and thus shunted to GND via resistors R1 and R2. The resulting voltage drops across R1 and R2 are given by the channel impedance. This impedance varies when a particle or a living cell passes the electrode area. An analysis at multiple frequencies at the same time (which is supported by the HF2IS and the HF2CA) allows for concurrently analyzing cell size and dielectric properties. With this information biologists, e.g. sort their cells and detect cell viability or health.

Impedance Measurement

hf2 application impedance
Figure 7. Measure an impedance using the HF2CA

The HF2 in conjunction with the HF2CA can be used to measure impedances at various frequencies. The connection diagram is shown in the figure above. The impedance of interest, Z, is connected to the input resistor in the HF2CA preamplifier. For optimal signal-to-noise, the input resistor, R1, is set to a value close to the impedance Z. The HF2 generates an output signal of amplitude VOut and the output signal from the preamplifier is connected to the positive input (Input +) of the HF2, which is here called VIn. With this setup, the impedance Z can be calculated using the following equation:

\(Z=R(V_{Out} V_{In})/V_{In}\)

Here we neglected the output resistance, R2, of the HF2 device. The is valid as long as Z>>R1=50 OMEGA. Furthermore, at high frequencies the parasitic capacitances Cp1 and Cp2 will have to be included in the calculation. At even higher frequencies, Cp1 and Cp2 will be dominant.

Custom Input Impedance

hf2 application custom trans
Figure 8. HF2CA custom input impedance

Sometimes it is useful to choose a special resistance value in order to optimize the signal to noise by, e.g. impedance matching. In this case, an external resistor can be used instead of using the standard values inside the preamplifier. All internal resistors need to be disconnected in this case, which can be done using the standard preamplifier user interface.

Cable Recommendation

Table 5. HF2CA cable recommendation
Function Connector / cable type Vendor / part number

SMB to BNC connection

SMB to BNC cable

BNC jack to SMB plug

Farnell / Newark 1351896

SMB to BNC adapter

BNC jack to SMB plug

Digikey ACX1386-ND

BNC jack to SMB jack

Farnell / Newark 4195930

Custom access or cable assembly


Cable type RG-174

Digikey A307-100-ND

Farnell / Newark 1387745

SMB to cable

SMB plug to RG-174 cable

Tyco Electronics 413985-1

Digikey A4026-ND

Farnell / Newark 2141206

BNC to cable

BNC plug to RG-174 cable

Tyco Electronics 1-5227079-6

Digikey A32212-ND