# HF2TA Current Amplifier Data Sheet

This chapter contains the data sheet of the HF2TA 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, information how to test the specified performance, and finally an extended recommendation for 3rd party cables and connectors.

## Key Features

• 50 MHz operation range

• 2 independent amplification channels with selectable AC/DC coupling

• Wide range of current gain settings (100 V/A to 100 MV/A)

• Impedance measurements from 1 μΩ to 100 MΩ

• Voltage output amplifier with selectable gain 1 or 10

• Very low noise and low input leakage

• Single connector for power supply and control

The HF2TA current amplifier converts 2 input currents to output voltages in a frequency range up to 50 MHz. This device is an active probe which can be conveniently placed close to the measurement setup. It supports most applications where a current must be converted to a voltage. The advanced design of the HF2TA ensures stability and a smooth operation over the entire frequency range. The HF2TA transimpedance current amplifier with the HF2 Series signal analyzers allows for very high performance measurements and insensitivity to interferences thanks to reduced parasitics.

Figure 1. HF2TA functional overview

## Specifications

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

Table 1. General
parameter description

dimensions

101 x 78 x 23 mm

weight

0.4 kg

storage temperature

-20 °C to 65 °C

operating temperature

5 °C to 40 °C

specification temperature

25 °C

connectors

3 SMA inputs female, 3 SMA outputs female, 1 RJ45 (no Ethernet)

Table 2. Specifications
parameter min typ max

positive supply voltage VDD+

12 V

13 V

15 V

negative supply voltage VDD-

-15 V

-13 V

-12 V

supply current

50 mA

60 mA

100 mA

frequency response

frequency range

DC

-

50 MHz

frequency range (AC coupled)

10 Hz

-

50 MHz

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

-

-

50 MHz

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

-

-

50 MHz

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

-

-

40 MHz

input

input current range

depends on R1, R2, G1, G2 settings

input current noise

depends on R1, R2, G1, G2 settings

input voltage noise (10 kHz)

-

7 nV/√Hz

-

input voltage noise (10 MHz)

-

5 nV/√Hz

-

input leakage current

-

2 pA

20 pA

input voltage offset compensation range

-10 mV

-

10 mV

input impedance range (Z // 15 pF)

50 Ω

-

70 kΩ

input bias voltage range

-10 V

-

10 V

input signal level (damage threshold)

-5 V

-

5 V

output

output voltage gain (G1,G2)

1

-

10

transimpedance gain (R1,R2)

100 V/A

-

100 MV/A

transimpedance gain accuracy (R1,R2)

-

±1 %

-

digital control interface timing

input high level

2.2 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

-

-

Figure 2. Digital control interface 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 also on the capacitive load on the output of the amplifier.

Table 3. Gain dependent parameters 1
input impedance setting bandwidth / 3dB cut-off maximum input current range (G=1) maximum input current range (G=10)

100 V/A

50 MHz

±10 mA

±1 mA

1 k V/A

50 MHz

±1 mA

±100 μA

10 kV/A

8 MHz

±100 μA

±10 μA

100 kV/A

1.5 MHz

±10 μA

±1 μA

1 MV/A

250 kHz

±1 μA

±100 nA

10 MV/A

25 kHz

±100 nA

±10 nA

100 MV/A

12 kHz

±10 nA

±1 nA

Table 4. Gain dependent parameters 2
input impedance setting input impedance maximum input current noise measured at

100 V/A

50 Ω

150 pA/√Hz

1 MHz

1 k V/A

50 Ω

15 pA/√Hz

1 MHz

10 kV/A

50 Ω

2 pA/√Hz

1 MHz

100 kV/A

100 Ω

500 fA/√Hz

100 kHz

1 MV/A

300 Ω

250 fA/√Hz

100 kHz

10 MV/A

1.6 kΩ

100 fA/√Hz

10 kHz

100 MV/A

70 kΩ

50 fA/√Hz

10 kHz

Figure 3. Casing dimensions of the HF2TA

## Functional Description

The HF2TA is an external current preamplifier for the HF2 Series instruments from Zurich Instruments. The preamplifier can be placed close to the signal source, which significantly improves the measurement quality due to less parasitics effects and to smaller interferences.

The two signal channels of the HF2TA can be used as separate current amplification channels, or alternatively, in differential mode connected to the differential input of the HF2 Instrument. The channels settings can be set independently.

Figure 4. Detailed block diagram

### Input and Output

Transimpedance stage: the HF2TA measures the current flowing at the two input terminals. The current amplifier uses a standard transimpedance stage to convert the current to a voltage output. The input terminal is matched to 50 Ohms to allow for proper impedance matching at high frequencies. At high current gains, or low input currents, respectively, the input terminal acts like a low-impedance virtual ground. The input impedance depends on the gain settings as described in the table above.

Voltage gain 1 or 10: the HF2TA offers a voltage gain of either 1 or 10 after the transimpedance amplifier. This allows to optimize the signal-to-noise at small amplitudes and high bandwidths. The transimpedance gain often has to be kept small in order to meet the required bandwidth. A voltage amplifier helps in this case to improve the measurement quality.

JFET input amplifiers: the HF2TA 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 -5 V to 2 V for each input which is also the common mode offset range.

Offset adjustment: the offset of the input amplifier can be manually compensated. For this purpose, disconnect any signal from the input of the current amplifier and measure the output voltage. Change the offset voltage until the output is close to zero. All remaining offset should now come from other sources (like offset current or leakage from the device under test).

AC vs. DC mode: a selectable switch after the input amplifiers allows the user to measure DC signals, or when this is not required, to select AC coupling with a cut-off frequency at around 10 Hz to remove the DC offset. When working in AC, make sure that the first amplifier is not saturating. This can be checked by switching to DC and gain 1.

Aux output: the HF2TA comprises a general purpose low-noise analog output. This output can be used as a power supply for, e.g., photo diodes. The photo diode is connected to the auxiliary output and the virtual ground of the input, no additional power supply is needed.

Signal shield voltage: the bias input connector can be used to apply a bias voltage to the signal shield. This can be used, for instance, to power a remote sensor over the signal shield without introducing an additional ground loop. If this option is not used, the signal shield should be conveniently grounded with the control setting "Shield Voltage".

### Power Supply and Remote Control

The HF2TA is designed for use with the HF2 Series devices. It has to be connected to the ZCtrl 1/2 connectors of the host device using a single Ethernet cable which provides both power and control signals. A standard straight-through (as opposed to cross-over) 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 the DAC and thereby adjust the correct settings.

## Applications

• Low-noise and high-speed current amplification

• Photo diode preamplifier

• Impedance measurement

• Semiconductor testing

• Impedance spectroscopy

### Recommended Settings

In order to get the maximum performance out of your HF2TA, the following guidelines should be followed.

• low and high input current measurement

The HF2TA gain setting should be selected properly in the measurement path. The gain setting can be set according to Table 3. As one can see, each input impedance and G setting has a maximum input current range specified. With each recommended input impedance and G setting, the maximum current will produce the maximum voltage swing of ±1 V at the output of the HF2TA. At this level the input digitizer of the HF2 input channel will run close to its full dynamic range which results in the optimal SNR.

• low and high bandwidth measurement

HF2TA is specified to work up to the 3dB bandwidth of 50 MHz. Nevertheless, care must be taken when selecting input impedance gain settings. Table 3 details as well the maximum 3dB signal bandwidth for each gain setting. For example, with an input current containing frequency components of less than 12 kHz in frequency, the maximum transimpedance gain of 100 MV/A can be selected. At 50 MHz, only 100 V/A of transimpedance gain is available. G=10 can also be selected as well if more gain is required at high input signal frequencies.

• minimize cross-talk and parasitics effects

With the measured impedance placed closely to the input of the HF2TA and the HF2 device, four point measurement setup can help to minimize parasitic effect as well as the noise pickups from the cable. Furthermore, using shielded cable can greatly reduce the high frequency noise pickups from the surrounding environment.

• avoid HF2TA instability

Since HF2TA is a negative feedback amplifier, its feedback loop stability can be sensitive to input capacitance, especially at low R settings. In order to avoid possible under-damped behavior (i.e. oscillation) in the measurement, it is recommended to use as high as possible the selected transimpedance gain R when measuring a capacitive circuit. A short cable to the HF2TA input can also help to reduce the parasitic capacitance seen at the HF2TA input.

### Photo Diode Amplifier with HF2LI

The HF2TA current amplifier is suited to read out the current from a photo diode. The following figures shows three possible ways to use the device. In the first option, the photo diode is grounded on one side and connected to the current amplifier on the other side. The recorded signal is amplified and sent to the HF2 Instrument.

The second option provides a solution when it is necessary to apply a bias voltage across the photo diode. For this purpose the auxiliary output of the HF2TA can be used. Voltages in the range of +/-10V and currents up to 10 mA can be delivered by this connector. Alternatively the bias can be provided by another voltage source.

The third option supports the drive of the photo diode by means of the shield of the signal cable. This shield can be conveniently driven by the HF2TA by shorting the auxiliary output to the bias input. This option permits the user to connect the remote sensor with one single coaxial cable and while avoiding to introduce a ground loop in the system.

All HF2TA settings can be conveniently programmed inside the graphical user interface of the HF2 Instrument.

Figure 5. HF2TA photo diode amplifier
Figure 6. HF2TA photo diode amplifier with single coaxial cable

### Impedance Measurement with HF2IS

Figure 7. Measure an impedance using the HF2TA

The HF2TA current amplifier can be used in conjunction with the HF2IS instrument to measure impedances in a very wide range at frequencies up to 50 MHz. The connection diagram in the figure above shows how the impedance of interest Z is connected to the input of the HF2TA. For optimal amplification versus bandwidth setting, the table in the specification section may be consulted. Three cases and applications need to be distinguished.

• Measuring an impedance Z > 10 kΩ

For large impedances it is possible to neglect the output resistance of the HF2IS Instrument and the input resistance of the preamplifier, thus the simple setup provides good accuracy. The HF2IS generates an output signal of amplitude VOut and the output signal from the preamplifier is connected to the positive Input 2+ of the HF2, called VIn. With this setup, the impedance Z can be calculated using the following equation:

$$Z = R \cdot G \cdot V_{Out} / V_{In2}$$

• Measuring an impedance Z < 10 kΩ

For small impedances and higher precision a four point measurement setup is required. For accuracy in the range of 1%, the voltage VZ can be measured directly by the second differential Input 1+ and Input 1- of the HF2. In this case it is important to select the high ohmic input impedance option (1 MΩ) as otherwise too much current is dissipated in the measurement instrument. Also the HF2 should be configured for differential measurement. The resulting impedance Z is calculated using the following equation:

$$Z = R \cdot G \cdot V_{Z} / V_{In2} = R \cdot G \cdot V_{In1} / V_{In2}$$

• Measuring impedances with high accuracy (all values of Z)

Four point measurement setup allows the most accurate measurement by taking into account simultaneously the current flowing through the measured impedance and the voltage drop caused by the current flow. For an accuracy better than 1%, it is recommended to use a voltage preamplifier with high-ohmic input stage to measure the voltage across the impedance VZ = VIn1. Assuming Vin2, R and G are the output, the resistor setting and the gain of the HF2TA, respectively, the resulting equation to calculate the impedance will be similar to the previous case:

$$Z = R \cdot G \cdot V_{Z} / V_{In2} = R \cdot G \cdot V_{In1} / V_{In2}$$ (assuming voltage pre-amp gain = 1)

A pictorial representation of how to set up the four-point measurement is shown below.

Figure 8. HF2IS four-point measurement setup

Note that the voltage measurement is made differentially through HF2CA then converted to single-ended input to the HF2IS while the current measurement remains single-ended throughout the current measurement path. Both HF2TA and HF2CA can be controlled using THE_LabOne UI. When they are connected through Ethernet cables to the back of the HF2IS instrument, LabOne will automatically add an HF2TA or HF2CA tab as shown in the screen shots below.

Figure 9. HF2CA tab
Figure 10. HF2TA tab

Both HF2TA and HF2CA can operate in AC-coupled or DC input. The HF2TA has a high-pass cutoff of 10 Hz while the HF2CA has a high-pass cutoff of 100 Hz. It is recommended that no R1 and R2 values are selected for the HF2CA to obtain maximum input impedance (i.e. no signal current loss through the HF2CA input ports) and therefore the most accurate current measurement.

## Performance Tests

In this section two tests are described that can be used to measure the DC leakage and the AC noise of the HF2TA. They can be performed by the user to do a sanity check on the validity of the measurement with HF2TA.

 Required equipment Specifications Recommended equipment HF2 Instrument No additional installation options required HF2LI or HF2IS HF2TA Current Amplifier HF2TA Specifications HF2TA Digital multimeter 0.1 mV Resolution, 20 V range Agilent 34410A SMA to BNC cables 2 x 50 Ω, male-to-male connectors supplied by Zurich Instruments Ethernet cable Category 5 or 6 supplied by Zurich Instruments

The following conditions have to be fulfilled:

1. The test equipment must be connect to the same AC power circuit. If you are unsure of the AC power circuit distribution, use a common power strip and connect all test equipment into it. Connecting the test equipment into separate AC power circuits can result in offset voltages between equipment, which can invalidate the verification test.

2. For accurate results, allow the test equipment to warm up for at least 30 minutes.

3. The HF2 Instrument as well as the HF2TA transimpedance amplifier are controlled by the LabOne software. Please make sure that the latest version of the LabOne software package is installed on the host computer. Please refer to the Getting Started for software installation instructions.

The HF2TA transimpedance amplifier has 2 analog input channels, 2 analog output channels and 1 external bias input and 1 auxiliary output. For the purpose of the following tests, the external bias input will not be used. The test setup for one channel is equally valid for the other channel.

### Input Leakage Test

#### Definition

This test measures the DC input leakage current of the HF2TA.

#### Setup

Figure 11. HF2TA DC input leakage measurement setup

The HF2TA is connected to the HF2 Instrument via the Ethernet cable for the purpose of configuring the HF2TA. The HF2 Instrument is not used for the measurement.

 Ch1 Offset (V) 0.0 V Ch1 R (V/A) 1 k and 100 M Ch1 AC OFF G 1 Input Shield GND Aux Output (V) 0.0 V

#### Measurement

The DC leakage current can be estimated by subtracting the inherent DC offset VO1 of the amplifier from the total offset VO2 due to both the internal offset and the input leakage. For this test the input of the HF2TA is left open. Then the output is measured with a digital multimeter as shown . The input offset VO1 can be estimated by setting the transimpedance resistor R to 1 k. The sum of the input offset plus leakage VO2 can be estimated by setting the transimpedance resistor R to 100 M. Then, the approximate leakage can be found by:

$$I_{leakage} = \frac{\left| V_{O2} - V_{O1} \right|}{100\ M\Omega}$$

### Input Noise Test

#### Definition

The noise generated by the HF2TA transimpedance amplifier itself can be expressed as input referred current noise. The following setup description enables users to verify through measurement if their HF2TA units have indeed the same noise level as specified in Table 4

#### Setup

Figure 12. HF2TA equivalent input current noise

For this test the HF2TA transimpedance amplifier input is left open. The goal is to refer the total contribution of out noise from the amplifier itself to the input and not from any other external circuits. Since the input of the HF2TA is left open, it is only necessary to define the sweep range in the HF2 instrument since no drive voltage is required.

The HF2 instrument settings for the test are given in the table below.

 Ch1 Signal Inputs Range auto range Ch1 Signal Inputs AC/ Diff/ 50 ON Ch1 Scale 1/R Filter BW setting type BW NEP (noise equivalent power BW) Filter BW 1 Hz

Notice that Scale has been set to 1/R where R is the HF2TA transimpedance value. This is to obtain the noise current referred back to input.

#### Measurement

The Sweeper will be used for the measurement with the following settings:

 Sweep Range Start 1 kHz Sweep Range Stop 50 MHz Sweep Range Points 50 Sweep Range Log Sweep ON

To have LabOne choose suitable filtering, averaging, and display settings, simply choose Noise Amplitude Sweep as the Application in the Settings sub-tab. Set Precision to High and start the sweep to measure the noise over the specified frequency. After division by the HF2TA transimpedance gain, the result can then be compared to the values in the input referred noise table.

## Cable Recommendation

Table 9. HF2TA cable recommendation
Function Connector / cable type Vendor / part number

SMA to BNC connection

SMA to BNC cable

BNC jack to SMA plug

Digikey J3606-ND

BNC jack to SMA plug

Digikey J10098-ND

Farnell / Newark 4195930

BNC plug to SMB plug

Farnell / Newark 1654647

Digikey ACX1324-ND

Custom access or cable assembly

Cable

Cable type RG-174

Digikey A307-100-ND

Farnell / Newark 1387745

SMA to cable

SMA plug to RG-174 cable

Digikey A32326-ND

Farnell /Newark 2112459

BNC to cable

BNC plug to RG-174 cable

Tyco Electronics 1-5227079-6

Digikey A32212-ND

Farnell / Newark 1831701